From the manual-
3 Channel--
1. 3 Channel mode cannot be enabled if only one or two DDR3 memory modules are installed.
2. When enabling 3 Channel mode with three, four or six modules, it is recommended that memory
of the same capacity, brand, speed, and chips be used.
When enabling 3 Channel mode with three memory modules, be sure to install them in the
DDR3_1, DDR3_3 and DDR3_5 sockets.
When enabling 3 Channel mode with four memory modules, be sure to install them in the
DDR3_1, DDR3_2, DDR3_3 and DDR3_5 sockets.
Hope this help's.