Well not every CPU and mobo can handle triple channel. Assuming yours can, have you checked the mobo installation guide to make sure you put the sticks in the appropriate channel slots? I suppose you could also be experiencing bad mem sticks, so 1 or 2 are dead/dying and therefore it goes to 1 channel cuz only one is active. To find out try running Memtest86+ (you burn it to a CD then boot off it). If it passes you should be ok, and I can only say the likely culprit is a BIOS setting.