Performance of 3 sticks of ram

Solution
It will operate in single channel and dual channel. However your current RAM may or may not work with the new stuff. The only way to find out is to try it.
You can use the built in "performance monitor" that is offered in control panel. Excuse me if this isn't the correct name- I recently came back to windows- but I know it still exists. This will tell you how much of your RAM you are using for different tasks. For instance- open up a highly taxing program or simply run a performance test;
if it goes above the "limit" of 1 stick then you're clearly utilizing at least two,
if it goes above the "limit" of 2 sticks then you're clearly utilizing all three =)

Hope this helps!
 
True, you always want the sticks to be the same brand/make to decrease any possible issues - but even with different brands and kits you can get it to work (if you're willing to put in some time playing with it to save on costs).