I think the difference is.. let's say, in a movie,
A) "flash-forward" means a sudden cut to the future. For example, the scene now shows A taking the exam, feeling anxious. This scene is immediately succeeded with the scene in which A is holding his F-graded exam paper. All the procedures and details between the two scenes, between the moment of taking the exam and the 'future'(the release of the result) are skipped in a flash.
B) "fast-forward" is the increase in the speed or the pace (the "2x", "4x", "8x" ) of showing the events afterwards. Unlike "flash-forward", you get to see all the events happening after 'the present' for 'fast-forward', only in a faster pace while "flash-forward" is a big jump afterwards, skipping all that in between.
C) "flashback" is the antonym for 'flashforward'. It therefore means a big jump forward to the past (instead of afterwards to the future).