You didn't waste your money, but definitely made a mistake. You should have ordered either 2 sticks or 4 sticks of memory. With 3 sticks, your system will operate in single channel mode, which is slower than the preferred dual channel mode you get when the sticks are paired up evenly.
My recommendation is to either just use 2 sticks of memory and keep the third as a backup, or buy a 4th stick of memory to make a full dual channel setup.
One final question,
You said that if I do utilize all three sticks of RAM then it will go from dual-channel to single-channel and single-channels are slower.
I did a little checking on Tom's Hardware and I found this benchmark which measured both single channels and dual-channels.
For gaming, there seems to be slight to nearly zero changes in performance. Nothing above 5%.
For such a small change, and I'm only going to be using this PC for gaming, nothing that would require RAM intensive work, what is your take on this situation?
Should I still just use 2 sticks of RAM or should I force it into single-channel?