Can/Should I run 3 RAM sticks

I think you got the answer from this thread. http://www.tomshardware.com/answers/id-2618149/compatibility-memory.html

You can mix the RAM, but there are no guarantees they will work together. 8GB is more than enough but 12GB is OK too, if they work together. If you use 3 then you are running the single channel mode.
Here is the single vs dual performance http://www.gamersnexus.net/guides/1349-ram-how-dual-channel-works-vs-single-channel/Page-3
4GB vs 8GB vs 16GB http://techbuyersguru.com/Ramgaming2.php