Something doesn't sound right. While it is common to have more than one of a color, there should the same number of each color. If all 3 blacks were connected to the brass terminal of the outlet, you could add a short length of black wire and connect it and 2-3 of the original ones with a wire nut. Then connect the new wire to the outlet.
This sounds like a three way switch. I had a similar problem and I thought that I had the wrong switch, but after I attached one wire on the left hand side and two on the right hand side, it worked quite well. Hope this helps. Good Luck.