Heather of SH3 IS supposed to be Cheryl of SH1. Of course, nothing has confirmed that Sharon (of the first movie), but it's entirely possible that Sharon survived the ordeal of the first movie, and that Rose is trapped in the otherworld with Alessa (and not the combined souls of Alessa and Sharon).
It is possible that Heather = Sharon. At the same time, that would require Christopher (Sharon's father) to take the alias of "Harry Mason" (which would be INSANELY confusing, considering that Harry Mason is Heather's father's REAL NAME, but whatever).
The rest of the cast is pretty optional. Rose is entirely possible considering that Heather could easily find her with Alessa. I don't see why Dahlia, despite her lack of any existence in SH3, can't be included. I'd be very odd to though.
Other than that, Alessa is most likely to return.