Build a triangle without any triangles

Ruby, 79

->n{1.upto(n*=3){|i|puts (' '*(n-i)).ljust(n+i,'/__\_\/\ \__/_/\/ '[i%3*6,6])}}

A. (-4 bytes, -1 +1) changed from 0-indexed (.times) to 1-indexed (1.upto)

B. (-5 bytes) changed from array of three 6-char strings to selection of 6-char substring of 18-char string.

C. (-1 byte) m=n*3 -> n*=3

D. (-5 bytes) reduced all five double backslashes to single backslashes (partly made possible by reordering of string required for point A)

Ruby, 94

->n{(m=n*3).times{|i|puts (' '*(m-i-1)).ljust(m+i+1,[ '/\\ \\__','/_/\\/ ','/__\\_\\'][i%3])}}

explanation

The basic unit is a 3x6 diamond as follows (last character on each row duplicated for clarity:)

    /\ \__/
   /_/\/ / 
  /__\_\/

All we need to do is display a suitable window of this pattern. Ruby's ljust allows you to pad with any string, not just spaces. Normally ljust would be used to pad a string of printable characters by adding spaces at the end, but here we use it in reverse: to pad a string of spaces by adding printable characters at the end.

ungolfed in test program

f=->n{
  (m=n*3).times{|i|                  #for each line of the triangle
    puts (' '*(m-i-1)).              #print m-i-1 spaces, 
      ljust(m+i+1,[ '/\\ \\__',      #left justified and padded to total length m+i+1
                   '/_/\\/ ',        #by one of these three strings
                  '/__\\_\\'][i%3])
  }
}

f[gets.to_i]

CJam, 47

ri_"/__\_\/_/\/ /\ \__"6/f**eeW%{_S.*s\~,\-<N}/

Explanation:

ri_       read the input, convert to integer and duplicate
"…"       push that string, containing the repeating pattern
           (3 lines in reverse order, concatenated)
6/        split into (3) lines of 6 characters
f*        multiply (repeat) each line n times
*         repeat the array of 3 lines n times
           at this point we have an array of 3*n strings with 6*n characters each
ee        enumerate the array (obtaining an array of [index string] pairs)
W%        reverse the array
           (so the lines are in correct order and indices in reverse order)
{…}/      for each [index string] pair
  _       duplicate the pair
  S.*     vectorized-multiply with " "
           this effectively replaces the index with a string of <index> spaces
  s       convert the pair to string, effectively concatenating the spaces
           with the string
  \       swap with the other copy of the [index string] pair
  ~,      dump the index and string on the stack and get the string length
  \-      subtract the index from it - this is the desired line length
  <       cut the concatenated string to that length
  N       add a newline

Try it online

Retina, 150 122 118 bytes

The output to this challenge looks awesome, by the way!

Input is in unary. The output contains a trailing linefeed. The code uses ISO 8859-1 encoding. Note the trailing space on the penultimate line.

(?=_\\¶.*1)
_\/__\
(?=/_/\\¶.*1)
/_/\/ 
(^|__)(?=/\\¶.*1)
$1/\ \__
ms}`(.*1*)1
/\¶/_/\¶/__\_\¶$1
m`^(?=(.*¶)*.)
$#1$* 

Try it online

Explanation

If you want a more in-depth explanation, comment or message me in chat.

(?=_\\¶.*1)                     # Matches the start of the 3rd line of every triangle
/__\_\                          #   and prepends another layer if needed
(?=/_/\\¶.*1)                   # 2nd line of each triangle
/_/\/ 
(^|__)(?=/\\¶.*1)               # 1st line of each triangle
$1/\ \__
ms}`(.*1*)1                 # This and above in a multi-single-line loop.
/\¶/_/\¶/__\_\¶$1               #   This stage adds a flattened triangle on top
m`^(?=(.*¶)*.)                  # Prepend a space for every line following -1
$#1$* 

Thanks to Martin for golfing 32 bytes off.