I would not say that BG had more multiclassing options than DnD 3E
In BG there was a limited selection of multi classes (or dual class for humans) that was restricted by race and alignment. Multi class characters could not select subclasses.
In DnD 3E you could giva a char any class you like at any level up, only restricted by alignment and very few race restrictions (arcane archer, dwarven defender)
In DnD 5E any char of any race or alignment can chose any class. If they allow multi classing ( I hope they will) you can select any class at every level up.
The IE 2nd ed. system was really stupid. Dual classing was useless apart from a few overpowered options while multi classing was usually better than single classes apart from a few overpowered kits. It was not a good or balanced system.
