my $brackets = qr(\[[^\]]*\]); # text enclosed in brackets
my $char = qr([^\[\]\s]); # not spaces or brackets

s{
  (                # group to $1
    (?:
       $char          # a char 
       (?:$brackets*) # followed by any number of brackets
    )
    {3}               # 3 times
  )
  (?!(?:$brackets*)(?:\s|\Z)) # ADDED: eliminates '-' on the end of words w/ a multiple of 3 chars
  (?=              # looking forward to ensure that
     [^\[\]]*      # we are not inside of a brackets
     (?:
       \[
       |
       \z
     )
  )
}
{$1-}gx;

print;