Your RAM is likely running dual-channel with four sticks loaded.
If you choose to go with 3x2GB, they are likely supposed to go in the lighter-colored slots. You can confirm this in your mainboard manual. They would have info like that very prominently displayed.
With all six slots loaded with identical RAM, it would be triple-channel.
Edit: I downloaded the manual. Apparently you can run four sticks in a weird way. You would need to use A1, A2, B1, C1. You would have three sticks in triple-channel and one stick in single-channel mode. If I was you, I would just use three sticks of that RAM. Or, if you think you could use 12GB, get another two sticks of it and fill all six slots.