The answer is YES
Do not install non even number of ram.
This will cause ALL your ram to run at half speed.
Insert them in pairs..
If you really want to do then you can put it in any available slot. Does not matter, and you can mix and match makes- because it is fairly generic ram, with generic timing and speeds.
Using 3 stick might cause your boot screen to beep, or moan that the ram is running half duplex, is uneven or might even show 400mhz instead 800mhz. That is normal. When you get another stick, it will get back to normal, but make sure they are all 800mhz
If you are putting in specific ram with boosted performance. then all 4 need to be the same so you can get the best performance.