The regular expression:

(?-imsx:(?=(\d\d\d)))

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (?=                      look ahead to see if there is:
----------------------------------------------------------------------
    (                        group and capture to \1:
----------------------------------------------------------------------
      \d                       digits (0-9)
----------------------------------------------------------------------
      \d                       digits (0-9)
----------------------------------------------------------------------
      \d                       digits (0-9)
----------------------------------------------------------------------
    )                        end of \1
----------------------------------------------------------------------
  )                        end of look-ahead
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------