I currently have a three-way switch controlling 2 sets of outside flood lights. I want to add a third set of outside flood lights. What is the easiest way to wire this third set? Do I need three conductor wire going to this light fixture?

How far are you installing the 3rd set from the others? Can you not just tap into one of the other lights and run to the 3rd set of lights? That way you can keep everything as it is.

I thought about tapping in to one of the other lights and I believe I can. I already ran 12/2 w/grd to the light but was hoping I didn't need 3 conductors going to the light.

What do you mean, you have already run 12-2? From where to where? You only need 12-2 from light to light. If you are fortunate enough to be wired as shown below, you can run 12-2 from one of the switches as shown but only from that switch.

Thanks HK. I already ran 12/2 to the new light and didn't know where I had to tie it in but your drawing explains it all. Thanks again.