I have two lights controlled by the same two 3 way switches, how do I go about adding a third light to the run? Thank you.

The easiest way is simply to run a cable (2 wires + ground) from the box of one of the two lights to the third light location. Wire it in parallel (black-black, white-white) with the existing light.

The switches control power to the black wire.

If you cannot do that, look at the two switches. Each switch has a unique terminal (usually dark in color). The LINE voltage comes into the unique terminal of one of the switches, and the unique terminal of the other switch goes to the lamps. You could tap off the wire that goes to the lamps (but be careful, there may not be a neutral in the switch box and you need to run that to the new lamp, too).