Some people have succesfully ran triple channel in dual channel boards, but with varying degrees of success.
I'd advise you against it. Dual channel mobo = dual channel RAM. triple Channel mobo = triple channel RAM.
Triple channel kits are desinged to work in threes, hence triple channel.
Someone on another forum had run triple channel 1600mhz RAM on a dual channel mobo, and he could just about manage 1587mhz with his RAM, before problems started appearing. But even at 1587 mhz he wasn't 100% stable, with random lockups and freezes.