Batch, 120 bytes

@set N=^&echo.
@echo     _ _ _%N%   /_/_/_/\%N%  /_/_/_/\/\%N% /_/_/_/\/\/\%N% \_\_\_\/\/\/%N%  \_\_\_\/\/%N%   \_\_\_\/

Japt -R, 43 bytes

`816cj6d9j6ÉrnÌ£nfea1u`qa ËnH" _/\\"Ãû

Test it

AWK, 110 bytes

$0="    _ _ _\n  "(a="/_/_/_")(b="/\\")"\n "a b b"\n"a b b b RS (c="\\_\\_\\_")(d="\\/")d d"\n "c d d"\n  "c d

Attempt This Online!

$0=                    # set default output
"    _ _ _\n  "        # top line
(a="/_/_/_")           # top left squares
(b="/\\")              # top right squares
"\n "a b b"\n"a b b b  # repeat above
RS                     # newline
(c="\\_\\_\\_")        # bottom left
(d="\\/")              # bottom right
d d"\n "c d d"\n  "c d # repeat

tinylisp, 246 bytes

(d K string
(d L disp
(d g(q((S N)(i S(g(s S 1)(c 32 N))N
(d f(q((S N)(g S(chars N
(L(K(q (32 32 32 95 32 95 32 95
(L(K(f 2(q /_/_/_/\
(L(K(f 1(q /_/_/_/\/\
(L(K(f 0(q /_/_/_/\/\/\
(L(K(f 0(q \_\_\_\/\/\/
(L(K(f 1(q \_\_\_\/\/
(L(K(f 2(q \_\_\_\/

Try it online!

Hmm. tinylisp does have some minimal string handling, but handling spaces is tricky!

Zsh, 86 bytes

Port of the PHP solution. Try it online!

sed 's|0|/_/_/_/|;s|1|\\/|g;s|2|\\_\\_\\_|'<<<'   _ _ _
  0\
 01\
011\
2111
 211
  21'

But the naïve approach only costs 1 extra, for 87 bytes ...

 <<<'    _ _ _
  /_/_/_/\
 /_/_/_/\/\
/_/_/_/\/\/\
\_\_\_\/\/\/
 \_\_\_\/\/
  \_\_\_\/'

Vyxal j, 33 32 bytes

^{-1 bytes thanks to Aaron Miller}

»\!¡øoΠ¥⅛831‟½ø↲p∇±₀WṀ»‛_ k/+τ7/

Explanation

»\!¡øoΠ¥⅛831‟½ø↲p∇±₀WṀ»             - Push a base-4 number containing the cube
                       ‛_ k/+τ      - Change to custom base, replacing the numbers 
                                      with the symbols.
                              7/    - Divide into seven lines.
<j flag>                            - Join by newlines
<implicit output>                   - Print it!

Try it Online!

Self-modifying Brainfuck, 86 bytes

Replace the ␀ with an actual null byte (0x00).

<[.<]␀/\_\_\_\  
/\/\_\_\_\ 
/\/\/\_\_\_\
\/\/\/_/_/_/
\/\/_/_/_/ 
\/_/_/_/  
_ _ _   

Try it online!

Explanation

Another literal output answer.

<[.<]             - Print the source code in reverse until a NULL byte is found.
     ␀            - Null, replace with 0x00. Ends program.
      <the rest>  - The cube in reverse.

Deadfish~, 687 bytes

{iii}iiccc{iiiiii}iiic{dddddd}dddc{iiiiii}iiic{dddddd}dddc{iiiiii}iiic{{d}ii}dddddc{ii}iicc{i}iiiiic{iiiii}ddc{ddddd}iic{iiiii}ddc{ddddd}iic{iiiii}ddc{ddddd}iic{iiii}iiiiic{{d}ii}ddc{ii}iic{i}iiiiic{iiiii}ddc{ddddd}iic{iiiii}ddc{ddddd}iic{iiiii}ddc{ddddd}iic{iiii}iiiiic{dddd}dddddc{iiii}iiiiic{{d}ii}ddc{iiii}dddc{iiiii}ddc{ddddd}iic{iiiii}ddc{ddddd}iic{iiiii}ddc{ddddd}iic{iiii}iiiiic{dddd}dddddc{iiii}iiiiic{dddd}dddddc{iiii}iiiiic{{d}ii}ddc{{i}dd}iiciiicdddciiicdddciiicdddc{dddd}dddddc{iiii}iiiiic{dddd}dddddc{iiii}iiiiic{dddd}dddddc{dddd}iiic{ii}iic{iiiiii}ciiicdddciiicdddciiicdddc{dddd}dddddc{iiii}iiiiic{dddd}dddddc{dddd}iiic{ii}iicc{iiiiii}ciiicdddciiicdddciiicdddc{dddd}dddddc

Try it online!

Perl 5, 89 bytes

say"   _ _ _";$r[7-$_]=($r[$_]=$"x(3-$_).'/_'x3 .'/\\'x$_.$/)=~y|/\\|\\/|r for 1..3;say@r

Try it online!

COBOL, 238 bytes

Compiled with open-cobol. Note that the indentation is a single tab, not spaces, even if this website formats it that way.

    IDENTIFICATION DIVISION.
    PROGRAM-ID. a.
    PROCEDURE DIVISION.
    DISPLAY "   _ _ _".
    DISPLAY "  /_/_/_/\".
    DISPLAY " /_/_/_/\/\".
    DISPLAY "/_/_/_/\/\/\".
    DISPLAY "\_\_\_\/\/\/".
    DISPLAY " \_\_\_\/\/".
    DISPLAY "  \_\_\_\/".
    STOP RUN.

Charcoal, 28 23 bytes

Ｆ³⁺⁺×\_³×\/⁻³ι¶ ‖Ｍ↑×_ ³

Try it online! Link is to verbose version of code. Not very Charcoal-y I know. I didn't realise that ‖Ｍ↓ deliberately moves the cursor so I had originally tried Ｆ³«Ｐ⁺×/_³×/\⁻³ι↗»‖Ｍ↓×_ ³ which didn't work, because I was trying to do it upside-down. If you want something more Charcoal-y then try Ｇ↗³←⁶↘³/\¶\/Ｇ↖³←⁷↘³_\¶\_↘‖Ｍ↑×_ ³ for 32 bytes: Try it online!

Brainf*ck 387 bytes

Not even sure if this is still a thing but I was bored and did it for sh*ts and giggles :)

++++++++[->++++>++++>++++>++++<<<<]+++++[->>+++>+++>+++<<<<]<+++[->+++++<]>[->>>+++>+++<<<<]>>>>+++<<<...<+++[->.>>>.<<<<]<<++++++++++>+++++++++++++.<.>>>..>.<<+++[->>>>.<<.<<]>>>.<<<<.<.>>>.>.<<+++[->>>>.<<.<<]>>>.<.>.<<<<.<.>>>>.<<+++[->>>>.<<.<<]>>>.<.>.<.>.<<<<.<.>>>>>.<<<+++[->>>>.<.<<<]>>.>.<.>.<.<<<.<.>>>.>>.<<<+++[->>>>.<.<<<]>>.>.<.<<<.<.>>>..>>.<<<+++[->>>>.<.<<<]>>.<<<.<.>>

Edit: TIL: I am 54.28% more effective than some generator I found online ^.^

Edit2: Try it online Make sure Wrap is on for Memory overflow behaviour in the options

Windows Batch, 122 bytes

@echo    _ _ _
@echo   /_/_/_/\
@echo  /_/_/_/\/\
@echo /_/_/_/\/\/\
@echo \_\_\_\/\/\/
@echo  \_\_\_\/\/
@echo   \_\_\_\/

Pretty self-explantory.

05AB1E, 35 32 31 bytes

"/ _\"•₁7æ¤ÝI}?ÕR&Ü•4вè4ôJ€∞ø¨»

Try it online!

100% different method than Emigna.

Edit: If I start with the pattern already transposed I can cut out 3 bytes.

"/ _\"            # Push ASCII-art characters used.
•₁7æ¤ÝI}?ÕR&Ü•4в  # Push transposed version of base-4 pattern.
è                 # Replace all digits in base-4 number with appropriate symbol.
 4ô               # Split into groups of 4.
   J              # Join together.
    €∞            # Mirror each row (technically column).
      ø           # Transpose back.
       ¨»         # Remove the extra "_ _ _", and print with newlines.

CSS, 225 223 bytes

-2 bytes thanks to Stephen S, extra spaces removed

I'm not exactly sure if this counts because CSS isn't really a programming language, but technically CSS can be standalone since the <html> element is autogenerated if there isn't one.

html:after{content:'   _ _ _ \A   /_/_/_/\005c\A  /_/_/_/\005c/\005c \A /_/_/_/\005c/\005c/\005c \A \005c_\005c_\005c_\005c/\005c/\005c/ \A  \005c_\005c_\005c_\005c/\005c\/\A   \005c_\005c_\005c_\005c /';white-space: pre;}

And here's a version with a monospaced font, 247 246 bytes.

-1 byte thanks to Stephen S, extra spaces removed

html:after{font-family:'Courier';content:'   _ _ _ \A   /_/_/_/\005c\A  /_/_/_/\005c/\005c \A /_/_/_/\005c/\005c/\005c \A \005c_\005c_\005c_\005c/\005c/\005c/ \A  \005c_\005c_\005c_\005c/\005c\/\A   \005c_\005c_\005c_\005c /';white-space: pre;}

Ruby, 72 70 bytes

7.times{|i|puts' '*(j=i^i/4+3)+'_ \_/_'[-i/3*2,2]*3+'/\/'[i/4,2]*j^=3}

Latest edit: avoid double \ by enclosing strings in '' instead of "".

Ungolfed (note in Ruby negative string indexes wrap around. -1 is the last character.)

7.times{|i|                       #7 lines.
  puts" "*(j=i^i/4+3)+            #Print j= 3,2,1,0, 0,1,2 spaces.
      "_ \\_/_"[-i/3*2,2]*3+      #Print 3 repeats of 2 characters from the string, index 0,-2,-2,-2,-4,-4,-4.
      "/\\/"[i/4,2]*j^=3          #Print 2 characters from the string, /\ or \/, repeated 3^j times.
}

Python 2, 81 bytes

i=7
while i:k=min(i,7-i);i-=1;print' '*(3-k)+'\/___ '[i/3::3]*3+k*'\//\\'[i>2::2]

Try it online!

Python 3, 98 94 bytes

@KevinCruijssen suggested this...

-4 bytes thanks to @HyperNeutrino and @WheatWizard

Output Hardcoding solution:

print(r"""   _ _ _
  /_/_/_/\
 /_/_/_/\/\
/_/_/_/\/\/\
\_\_\_\/\/\/
 \_\_\_\/\/
  \_\_\_\/""")

Try it online!

or, 98 bytes:

print("   _ _ _\n  /_/_/_/\\\n /_/_/_/\/\\\n/_/_/_/\/\/\\\n\_\_\_\/\/\/\n \_\_\_\/\/\n  \_\_\_\/")

Try it online!

Python 3, 139 bytes

Dumb Python 3 Non-Hardcoding sumbmission, sure it can be golfed. Surprisingly, compressing the two fors in just one loop is longer:

p=print;p(' '*3+"_ "*3)
for i in range(3):p(abs(2-i)*' '+'/_'*3+'/'+i*"\/"+'\\')
for i in range(3):p(i*' '+'\\_'*3+'\\'+"/\\"*abs(2-i)+'/')

Try it online!

Python 3, 140 bytes

print(' '*3+"_ "*3,'\n'+'\n'.join(abs(2-i)*' '+'/_'*3+'/'+i*"\/"+'\\'if i<3else (i-3)*' '+'\\_'*3+'\\'+"/\\"*abs(i-5)+'/'for i in range(6)))

Try it online!

05AB1E, 34 31 bytes

„_ 3×Âð'/:3F„/\«∞2ä`RˆD}\)¯R«.c

Try it online!

Explanation

„_ 3×             # push the string "_ _ _ "
     Â            # push a reversed copy
      ð'/:        # replace spaces with slashes
3F                # 3 times do
  „/\«            # append "/\"
      ∞           # mirror
       2ä`        # split into 2 separate parts on stack
         Rˆ       # reverse the second part and push to global list
           D      # duplicate the remaining part
            }     # end loop
\)                # discard the last copy and wrap stack in a string
  ¯R              # push the global list and reverse it
    «             # append to the rest of the list
     .c           # pad lines to equal length

Alternate 31 byte solution

„ _3×3FDð'/:„/\«∞2ä`R.Á})ÀÀÀ.c¦

JavaScript (ES6), 95 bytes

_=>String.raw`   _ _ _
  /_/_/_/\
 /_/_/_/\/\
/_/_/_/\/\/\
\_\_\_\/\/\/
 \_\_\_\/\/
  \_\_\_\/`

JavaScript (ES6), 103 94 bytes

-9 bytes thanks to @Shaggy

_=>`   _ _ _
  ${a=`/_/_/_/\\`}
 ${a}/\\
${a}/\\/\\
${b=`\\_\\_\\_\\/`}\\/\\/
 ${b}\\/
  ${b}`

JavaScript (ES6), 106 bytes

_=>`   _ _ _
  /_/_/_/\\
 /_/_/_/\\/\\
/_/_/_/\\/\\/\\
\\_\\_\\_\\/\\/\\/
 \\_\\_\\_\\/\\/
  \\_\\_\\_\\/`

PHP, 77 bytes

<?=strtr('   _ _ _
  0\
 01\
011\
2111
 211
  21',["/_/_/_/","\/","\_\_\_"]);

Try it online!

Retina, 59 bytes

   _¶  /_/\¶ /_/V\¶/_/VV\¶\_VVV¶ \_VV¶  \_V
._
$&$&$&
V
\/

Try it online! Explanation: The first stage simply creates the following:

   _
  /_/\
 /_/V\
/_/VV\
\_VVV
 \_VV
  \_V

The second stage then expands all the _s by repeating them and the previous character 3 times, while the third stage turns the Vs into \/s.

><>, 97 94 93 89 bytes

">knknknk//>k>knknknk/>k>k>knknknkk>k>k>n>n>n>k>k>n>n>n>/k>n>n>n>//n/n/n///"01.
of-

Try it online!

Edit 3: Figured out that you can use the character "" (ascii 25) - Now the process just takes 15 off every stack item and prints. Also removed extra directional instructions and replaced all with 1 move (01.)

Tail, 99 bytes

#!/bin/tail -n+2
    _ _ _
  /_/_/_/\
 /_/_/_/\/\
/_/_/_/\/\/\
\_\_\_\/\/\/
 \_\_\_\/\/
  \_\_\_\/

C#, 86 bytes

o=>@"   _ _ _
  /_/_/_/\
 /_/_/_/\/\
/_/_/_/\/\/\
\_\_\_\/\/\/
 \_\_\_\/\/
  \_\_\_\/"

Try it here.

Java 8, 112 bytes

o->"   _ _ _\n  /_/_/_/\\\n /_/_/_/\\/\\\n/_/_/_/\\/\\/\\\n\\_\\_\\_\\/\\/\\/\n \\_\\_\\_\\/\\/\n  \\_\\_\\_\\/"

Hard-coded output.

Try it here.

Python 2, 93 bytes

print r"""   _ _ _
  /_/_/_/\
 /_/_/_/\/\
/_/_/_/\/\/\
\_\_\_\/\/\/
 \_\_\_\/\/
  \_\_\_\/"""

Try it online!

Bubblegum, 42 bytes

0000: e0 00 4f 00   22 5d 00 10   68 13 e2 04   15 00 b7 11 │ à.O."]..h.â...·.
0010: 7a 0e c5 f5   30 27 b5 b3   3d 39 8f a6   1f f9 74 52 │ z.Åõ0'µ³=9.¦.ùtR
0020: c5 66 98 bd   bd 0a 9a 8d   44 00                     │ Åf.½½¶..D.

Bubblegum, 35 bytes

00000000: 5350 5088 0741 2e05 05fd 7830 8ce1 82b3  SPP..A....x0....
00000010: 806c 0413 c889 8907 4330 8f4b 01c1 036a  .l......C0.K...j
00000020: 8671 00                                  .q.

Try it online!

SOGL V0.12, 27 bytes

Y=Q∫+ZΔ○H!╝Ηūf⅟Ξ∆׀Æģ≠a⁶‘6«n

Try it Here!

Sadly, the palendromization version qE½Dε▒2β[}█O%q‘6«n╬∑ doesn't really work