I'm currently on episode 2 and I'm not going to play the expansions right after I beat the main game, but I might consider them if they're good. So any opinnions? Worse or better than the main game.

I think you misunderstand what the second and third games are. They're not expansions to Redneck Rampage, but separate games, so that the three of them form a connected story. One thing that may confuse you, however, is that the second game installs over the first so that you can't play the first one any more. To avoid that, install the first installment into a different folder and install the second over that copy, and all will be well. And yes, all three games are well worth your time.

The gog version always ask what version do you want to launch, the base game or one of the other game's or expansions.

Anyways, thanks. Good to know that they are worth playing.

Redneck Rampage Rides Again is superb addition to the main game. Arguably even better than original 2 episodes from RR. Lots of memorable maps, dense atmosphere, great design all around. If you liked original RR you just have to play this expansion.
It probably helped that it was built by Xatrix themselves, so they expanded and improved on everything that made RR so good.

Suckin' Grits on Route 66 on the other hand is just okay, not bad but not outstanding either. So feel free to play it if you just want more of it. Though if you're a fan of RR it still worth playing. It was made by an external studio.