quote; If the two 2" then do I just plumb them in in series or do they have to have separate drains?
separate, otherwise you still only have one 2" drain
If the one 3" will I have to run a full 3" drain line all the way to the main drain line in the house?
Yes, because you cannot reduce the drain line in the direction of flow.
If two 2" how do I build a proper pan to slope it correctly?
Build a "ridge" down the center. Put a drain in the middle of each section, and slope to the drains.