Just did a little research on these three (set for a Feb. 2011 release), and it looks like KILLER PARTY might be the one I'll buy. I looked them up on IMDB and I may have seen it when I was a kid. The plot seems familiar, and it seems to be not just a slasher flick, for the killer is an evil spirit, he possesses some chick, killings ensue, etc. I might give this one a go.
Here's some trivia I found on IMDB about it, which I thought was interesting:
This film began filming in 1978, but when it was 90% done, the film was shut down due to going over-budget. The rest of the film was shot in 1984, but sat on the shelf for two years before receiving a limited theatrical release by MGM in an edited version that omitted most of the gore in the original cut.
When released in 1986 the title was changed from "The April Fools" to "Killer Party" because the distributors feared that it would be confused with the film April Fool's Day (1986/I) which was also released around that time.
UPDATE: Went back and looked at THE PACK again on IMDB. Hmmm, jury's still out on that one. Animal attack flicks creep my out, especially, if they're done realistically, and they say the attack scenes in this one are rather realistic. I'm tempted. Don't know yet.
UPDATE