“Multiclassing rules will follow closely the 5e DnD. On level up characters will be able to continue with their current class or choose a new class, provided they meet the requirements. Multiclassing is not going to be available in Early Access at launch, we're planning to add it later.”
From a Reddit AMA
https://www.reddit.com/r/BaldursGate3/comments/netrz6/multiclassing_on_release/Had multiple tabs up, wrong link:
https://www.reddit.com/r/IAmA/comments/fhk1u3/im_swen_vincke_creative_director_at_larian/Oh, you are specifically asking it the game will launch with it? That’s not confirmed but I wouldn’t stress about that.
