I currently have 2x4gb kingston ram sticks installed in my MOBO.
I am planning on getting another 4gb kingston of the same model as already installed in my pc stick.
I have seen many threads about single and dual channels in RAM, and don't understand much about pc memory, so I would like to ask whether adding a 3rd stick would put me in single channel, keep me in dual channel, maybe go to a triple channel, and will it affect the memory of my pc somehow? (efficiency and performance-wise).
Ram is sold in kits for a reason.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards can be very sensitive to this.
That is why ram vendors will NOT support ram that is not bought in one kit.
Although, I think the problem has lessened with the newer Intel chipsets. Still,
it is safer to get what you need in one kit.
Assuming your ram works, the first two sticks will operate in dual channel mode, and the odd stick will operate in single channel mode.
That is not a big performance hit, you will usually benefit from the extra ram more.
You don't have a Triple Channel motherboard. They have three or six memory slots.
By adding a third stick you will be in SINGLE CHANNEL MODE which will reduce the speed by which the memory can communicate to the CPU. This probably will reduce performance slightly but it depends on how fast your memory AND CPU are.
You can't be in Dual on two and Single on the other. All memory must be communicating at the same speed. In fact, your MOTHERBOARD MANUAL actually shows you accepted configurations which are usually ONE, TWO, or FOUR sticks but not three (and for one and two you can have issues if you don't use the recommended slots.)
If this is for GAMES, then 8GB is already plenty.
*You'd likely be SLOWING DOWN your system with no advantage.
You can try, but as Geofelt said, mixing DRAM is a hit or miss - DRAM sold in sets is tested so that all the sticks will work together, you can pull 10 sticks right off the assembly line and maybe only get one 4 stick group that will all work together, so it's taking a chance - if you do try and have problems give me a shout, can often get sticks to play nice with minor voltage or timing adjustments, no guarantee but have had good success