After your question I looked around the net for answers and there is a lot of conflicting information regarding 3 sticks of DDR and dual channel. The general consensus seems to be that your 2X2G will run on dual channel while your 3rd stick will run in single channel mode.
Personally I would remove the third stick and check performance. If you notice a difference using less memory you could always grab a 4th stick to insure dual channel with all sticks.
you can run all 3 sticks fine. the memory managment controler will set dual channel mode for 2 sticks and single channel for the last. 1 of the benefits of the amd mmu.
it auto detects the most efficient solution. you may even be able to get another stick and have all 4 sticks running dual channel, but that is up to your motherboard and if it will allow it. some will only allow dual channel on the first 2 sticks only, and will switch to single channel mode for all 4 if 4 sticks are used or if they go above the cpu's maximum rated ram speed. (the intel i7 does the same if you use ram faster than 1333)