I wonder if this is something they addressed in the final build? The timing is awfully tight in the demo..there's not a lot of opportunity to go raw b+3, f+3 in a string both because of positioning and timing. If you're gonna do it without meter, you need to do it after your first launch, or the juggle gravity will hit you to hard. Of the three in the demo, Batman's the best at it. WW and Lex basically have to land it off a special (WW spin, or Lex ex gravity).
If you absolutely must have it, use the cancels (b,b+m) until you get a better feel for it.