Would you drive a car that needs 4 wheels on the road with only 3 wheels? No you wouldn't, similarly, always populate the slots on a motherboard in pairs. So if you have two ram slots, populate one or both. If you have a motherboard with 4 slots, populate 1, 2 or 4, not 3. You're not going to get dual channel no matter what setup you run them in when having 3 sticks. Also I'm an advocate of not mixing and matching sticks of ram.
If you want more ram, go get a dual channel ram kit x2 and then populate the slots on the motherboard.