A great example of a 3 way involves the old "lurker" concept from 4e. Basically creatures designed to ambush.
So you start with a regular fight, nothing special. Then suddenly one of the creatures gets grabbed by the lurker monster, which now changes up the dynamic.
Another way to do it is to change up the terrain. The problem with 3 way fights is generally if one side shows weakness, its best for the other two sides to gang up on them and finish them off, THEN worry about the other side. One way to correct this is with terrain adjustment. Perhaps A and B are attacking C....but then C gets cut off by a rock slide or a wall spell or something. Now A and B battle it out for a bit until C is revealed again.
Last idea, area spells are great to use in such fights, as it becomes easier to hit both of your enemies, so there is less desire to focus fire on just one enemy.