The quantifiers (like {3}) make sense in regular expressions. You used one in the replacement part, though, which is a pure string (and even if it worked, where would the spaces be?). There is no easy mechanism how to introduce repetition into a string, you have to call a function:

s/int/ join ' ', ('int') x 3 /ge
[download]

YMMV, but I think plain s/int/int int int/g is clearer.