The regular expression:

(?-imsx:&(?!(amp;)|(lt;)|(gt;)))

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 not:
----------------------------------------------------------------------
    (                        group and capture to \1:
----------------------------------------------------------------------
      amp;                     'amp;'
----------------------------------------------------------------------
    )                        end of \1
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    (                        group and capture to \2:
----------------------------------------------------------------------
      lt;                      'lt;'
----------------------------------------------------------------------
    )                        end of \2
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    (                        group and capture to \3:
----------------------------------------------------------------------
      gt;                      'gt;'
----------------------------------------------------------------------
    )                        end of \3
----------------------------------------------------------------------
  )                        end of look-ahead
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------