Also check with the manufacturer for their manual on ram placement on the board.
Normally you would only get to run it in dual channel mode with the 2 x 2gb sticks in the proper slots.
That's why I refer you to the rof the board to see how they explain how to populate the slots and what advantages, if any would be gained by running the 3rd stick.
(edit) were all basically saying the same thing I agree with whs about using all slots, but then I know they sell a lot of 3 x 2gb kits, so I'm thinking you'd be better off with 2 x 2gb kits. As whs as suggested.