New Hard Drive

I got a new hard drive for Christmas and I want to hook it up to another drive I already have with a raid card so that it's one massive drive for my video editing. The problem I am having is that I already have 2 hard drives in my computer - one that came with that has all my programs and what not on it and the other one I put in to hold video files. There's not another slot for me to put another hard drive.

So the question I ask to all of you is: Is there any way I can make room for one more hard drive?? I want to connect the two big ones together and then keep the smaller of the three for everything else I do on my comp.