There's a lot more than 3 endings...there are endings exclusive to the origin characters too...take Gale for example...he has something like 3 or 4 endings you can only see if you play as him. And there are a lot more very major permutations on the main 3 endings just related to your character depending several factors.
Just some fairly generic descriptions:
Main character can end up the literal slave of one of the companions, having his mind erased and the body taken over by a mad god, doomed to become a monster, retiring to live with his lover, or with little left to live because empowered a devil to take over all of the nine hells and said devil is coming for the forgotten realms with nothing and nobody powerful enough to stop him. And that's probably not even all of it.
Not to mention there are a lot of secrets and plot twists you probably couldn't get in a couple playthroughs...like uncovering the real identity of the guardian(yeah he has a name and not an insignificant one at that), then there are at least two main story related plot twists you can only see if you play as the dark urge, and so on. This game will need at LEAST a dozen playthroughs, bare minimum, just to see all the important stuff.
