Are you running a 64bit operating system? 32bit Operating systems can't "see" more than around 4gigs of ram. Also, what type of motherboard do you have?
In order to run the RAM sticks in dual or tripple mode, they will need to be in certain slots. You'll need to read the manual for your motherboard to know which slots to go into, and whether or not your mobo supports tripple mode. If it doesn't support tripple mode, you might not be able to use the 3rd stick because it could cause your system to run in single mode. It depends on how your motherboard works.
Download this program to help you know more info on your system, it's a very nice little free program: