Posted March 12, 2016
If you've got a F, T, C, and F/T, then XP will be split 4 ways, and the F/T will tend to have a lower Fighter level than the F, while the F/T will also tend to lag behind the T in Thief level. The F/T gets to use both Fighter AND Thief abilities as the trade off.
At lower levels, XP requirements typically double at each level. Therefore, multi-class characters will only be one level behind their single-class counterparts.
At higher levels, typically starting in the 9-11 range (depending on class), every level requires the same amount of experience; hence, multi-class characters will only gain levels half as fast. For instance, at 3 million experience, a single class Mage will be level 18 (and hence able to cast 9th level spells), while a double class Mage (like Aerie) will only be level 14 (and hence will have just gained 7th level spells). This causes multi-class characters to be significantly worse than their single-class counterparts at these levels.
At still higher levels, the level gap continues to widen; however, levels stop mattering. After level 20, there isn't much to gain for being a higher level, except for HLAs, which multi-class characters acquire as quickly as single-class characters. In the meantime, multi-class characters get to catch up (although it is worth noting that triple class characters never get 9th level spells unless you use a mod to raise the XP cap (normally 8 million in ToB) to 9 million or higher).
