The manual (p. 16) shows that triple channel 3 sticks go in slots 1, 3, and 5. And that the BIOS will automatically find the RAM.
What is your BIOS?gigabyte shows
F3 2010/02/06 Enhanced Memory capability
F2 2010/01/29 Support Intel Core i7 6 core processors
F1 2009/12/28 First Release
If you are not using F3, consider flashing your BIOS to F3.
If none of this works, then consider returning the board for one that woorks.