I would install a wall switch.
- Turn the lights on.
- Flip breakers until the lights go off.
- Find a receptacle near the lights that also went off.
Now, chances are, that there is a cord running up the wall from that receptacle that powers the lights.
- Remove the receptacle. (Power still off!) The wire coming into the box from the bottom is likely your panel connection, and the one running out the top is probably your light power. BUT!!!! it might also be powering other things.
- Disconnect the upper wires from the receptacle, wire nut the ends so they're safe, and wrap electrical tape right around the screws on the receptacle so you don't accidentally short it out against the box.
- Turn the power back on. Test the receptacle and the lights, and anything else in the vicinity. If everything is good, the lights are off, and the receptacle works AND everything else in the house works. If not, turn the power off and re-wire everything the way it was.
The following is really only applicable if the situation is exactly as I described above. There are many possible configurations. For example, the power could be going to the lights, then to the receptacle. Or, there could be another receptacle or more lights being powered at the end of the chain. In either situation, you'll need to pull more wire and damage more plaster. At this point, I'd call in a professional.
The following instructions involve messing around with wiring. If you do not feel 100% confident in your abilities, Call a professional.
Turn the power off!
Follow the wire up the wall to a good switch height.
- With a drywall saw, cut a hole for your new switch and box. (Get an OLD WORK box).
- Tape some 14-2 wire to the disconnected wire in the receptacle hole. Pull it up to the switch hole, leaving a foot or more of excess. (it may be staple to a stud. Yank hard.)
- Cut the old wire, leaving about a foot or more below the hole.
- Remove about 8 or more inches of outer sheathing from both the light wire and the new wire. Thread them both into the box and tighten the clamps. Install the box.
From the wire you're going to discard, pull out about 4 inches of bare copper. There should be a grounding screw at the back of the box. Screw both grounds and the short piece to this screw. Attach the other end of the short piece to the green screw on the switch. When wrapping around screws, your curl should be clockwise, so the screw doesn't squeeze them out.
Strip the whites, and wire nut them together.
- Strip the blacks, and attach them to the other two screws on the switch.
- Install the switch and faceplate.
- Go back down to the receptacle, and wire the new wire back up, the way the old wire was.
- Reinstall the receptacle and faceplate.
- Power on. And all 3 lights should be on the switch.
If you want 3 switches, then it's a bigger job, and may be impossible without serious wall damage.
Essentially you'd get a 3-gang box, pull the wire up from the receptacle, and feed new wires to each light individually. That's better left for another question.