2D/3D AI Tips needed.

#1 Exactly what's the problem? The basic concepts are the same in 2D and 3D.

If you are worried about pathfinding then I would continue to use your 2D algorithm and just handle the different levels (if thats the way your world works) manually, that should be fine if you don't have to many stairways/lifts. ie to go from some point on level 2 to some point on level 3, find a path on level 2 to the nearest lift and then find a path on level 3 from the lift to the destination.

Depending on the algorithm you're using you may not have to change anything, any graph searching algorithm for example will work fine in 2D, 3D or nD.

Without knowing more about your particular world I can't think of any other problems that are specific to 3D environments.