it's still in pre-production but according to IMDb (which is often wrong) only maclachlan and a couple of other original castmembers are fully "onboard' for the season...
it looks like everyone else is just a one episode "cameo" which is quite disappointing.
My feeling is that as long as it has a compelling story that organically ties in with the first two seasons, it doesn't matter how many of the original cast return. As far as cameo appearances go, I certainly hope they aren't "showcased" or edited in such a way that it comes off as "Oh looky looky, there's so-and-so!!" They should just appear in an unassuming, matter-of-fact way so that the STORY takes precedence, not the actor's involvement in the scene.