Unless your motherboard supports triple-channel RAM (which is unlikely, as I don't even know if it exists :lol
, you'd actually see a decrease in performance when using 3 sticks of RAM, as it'll force the RAM to run in (3x) single channel mode. [strike]You'll be running in dual channel mode at the moment[/strike], or (2x) dual channel mode when using 4 sticks.
Best to use it in multiples of 2, in my opinion.
EDIT: Just read that you're using 1 x 8gb stick. As Eximo said, buying another 8gb stick would be the best choice.