Well, first you need to make sure the "extra ram laying around" are all compatible and the same speeds etc.
Second, not as Ghot mentioned not sure why you're trying to use three sticks, you should stick to 2 or 4 sticks and make sure they are in the correct DIMM slots if only using 2.
Are you getting any kind of errors or beeps from the mobo when trying to boot it with RAM installed, or it doesn't even POST or anything? As Ghot already said post the RAM and Mobo for extra clarification.