If it was just one rival party, I'd say to stat them as PCs, absolutely. That's a thrill to recognizing your rivals as using the same rules you are. It means they're your equals, it means you're facing the wrong end of the powerful abilities you usually get to throw around, it means their capabilities are both familiar and uncertain, and it means you know they'll be leveling up just like you are for the next encounter.
Only... three rivals parties is too many. It's too much work, it's too many for both you and the players to keep track of, and it dilutes the special nature of having NPCs with PC classes. So I'd suggest one of two things. Either you slim down your plans so that there's only one rival party, or you run is so that only one of the three is a capital R Rival party and the other two are of obviously lesser caliber. You can do a lot with a party that's "One capable Red Wizard and her henchmen (half of whom are zombies)" without having to stat the whole team as PC classes.