First, a few things:
1) This is for a Greyhawk campaign, where I consider eclecticism the norm.
2) I have no race-class restrictions (hate 'em).
3) I'm assuming all the usual multi-class / class-and-a-half rules as per the PHB.
4) For (human) three class combos, I allow any three of the following: fighter, rogue, wizard, cleric.
5) For two class combos, I allow any of the following as per the attached chart.
6) I left knight, paladin, and bard off the list, as I'm not inclined to allow them to multi-class (they seem too specialized for that).
Seem reasonable? Too restrictive? Too permissive? Any problems I'm not anticipating?
Thanks in advance for your thoughts.