I agree, a Freddy vs. Jason vs. Ash would be goofy, just because I don't think the "reality" Ash inhabits doesn't jive with the reality in which Freddy and Jason exist. They don't go together; it's like peanut butter and motor oil.
That said, if an F v. J. v A. movie makes it even 1% more likely that we'll get a fourth Evil Dead movie, I say bring it on.
(And I have to admit that if such a thing did come out, I'd probably break down and see it just out of curiosity. And to see Ash kicking some donkey.)