Dual Channel on 3 sticks

When running 3 sticks of ram, which dimm slots do you put the 2 to be in Dual Channel in? 2 and 3 or 1 and 3? Have 2x256 and 1x512 that I will be installing when the 512 stick arrives in the mail

Slot 3 got the 512 on my DFI(being the furthest from the CPU) all other arangments want not work.

i think abit needs dual in slots 2&3, and any additional ram goes in slot 1.....