Hi newcomer and welcome to the Tom's hardware forum.
Yeah, you can install 3 stick of RAM, two of the will run in dual-channel, and the other one will run in single-channel, but this affect the performance of the rig. That's why the RAM is usually install in pairs of 4GB, 8GB or 6GB (tri-channel)
Why would this affect performance? I mean obviously it wouldn't be as fast has having 4 sticks all in dual channel, but I only have 3 good sticks at the moment. If 2 of them are running dual channel, I would think having any extra memory in single channel would still be better than going to the hard drive.