A) If u want the tri channel, u need install the RAM in A1, B1 and C1 slots, ur current configuration A1 A2 only can give u a dual channel even if u install another RAM stick, so, u need change the stick of the A2 slot to the B1 slot and install another one with the same specs that u current RAM in the C1 slot.
B) If u buy two more kits, all ur RAM slot will be populated and in that point u will have two tri-channel set running in ur rig, but BE SURE that are the same specs even if u can the same model that ur current RAM.