I have not read all of the posts so if I repeat anything already said, Sorry Bout Dat!
99EXP,
If I read you correct you have two lights already in place and want to install one more.
OK, check the wattage of the first two lights and the wattage of the third light, also check the amp rating of the relay you now have installed.
Let say they are all 55 watts so that is 165 watts, divide that by 12 volts and you have... a drum roll please, 13.75 amp draw.
If the relay has an amp rating of twice that then you can wire the third light to the existing relay.
Problem - there is always a catch - the wire going to the lights has to be at least 12-gauge wire to handle the amperage you need.
Now remember we are assuming the lights are 55 watts each and not the 150 watts versions. If they are the 150-watt versions then you will need an additional relay and larger wire.
Two 150 watt lights = 300 watts of light divided by 12 volts = 25 amps. 10-gauge wire and a 30 amp rated relay are just barely enough. NO SAFETY FACTOR !!!
Not as easy as you thought, huh!
Get a list of all of the ratings of the lights, relay, and wire size you now have and let us know; then the forum can make a good suggestion.