Won't this be a moot point once the full game launches... because I'm praying to the old gods that we "do not" have the painful BioWare stamp collection of companions following us around (meta gaming which sod gets the gift)... and instead have to make the hard choices of actually choosing who we keep for the whole game more or less (like old BG 2)... or even have companions just leave due to alignment issues. So hoping selection panel "is not needed".
Probably not, because you can theoretically recruit mercenaries and thus have more than 3 companions. There also might be official companions in Act 2.
I suppose BG3 could not allow you to hire mercenaries if your party is full, but that would be a terrible decision imo. D&D is a class-based system, so what if you happen to have no cleric or druid in your surviging set of companions? Do you then just not have a Divine Caster for the rest of the game?