We don't know how multiclassing will work in BG3...we know that it will not work as it does in D&D 5e...they are doing some funny stuff, especially with multiclassing into spellcaster classes. It seems as though they are trying to separate the levels of each class individually and correlate them more to player level as opposed to how many levels of a class you took before but we really don't know. Don't plan anything to do with multiclassing at all yet because it probably won't work as you expect. This will really not be the same as 5e at all.
