(?-imsx:^((?:[^_]+_){2}[^_]+)_)

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):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    (?:                      group, but do not capture (2 times):
----------------------------------------------------------------------
      [^_]+                    any character except: '_' (1 or more
                               times (matching the most amount
                               possible))
----------------------------------------------------------------------
      _                        '_'
----------------------------------------------------------------------
    ){2}                     end of grouping
----------------------------------------------------------------------
    [^_]+                    any character except: '_' (1 or more
                             times (matching the most amount
                             possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  _                        '_'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------