The funny part is that two of BG2's problems (no evil thief, available evil companions only fill 60% of the party slots) would be solved if they just brought back Xzar and Montaron. But apparently it was more important to implement bland as fuck companions like Valygar and Cernd.
In theory, if you take Edwin, Korgan, Viconia - and then allow Imoen to come back at your side in Spellhold, the remaining slot would be left for Sarevok in ToB - and thus, completing the party.
The evil thief conundrum was probably deliberate to make you care for Imoen despite rolling a selfish prick.
Ironically, you can get there with just them 3 because they're pretty much OP, and that includes diving your reputation into the ground by murdering all the NPCs everywhere, after peaking at 18 to get the best prices at the vendors you can't steal from AND face-tanking every trap.