Please help! I just installed 2 new 3 way switches and a new light. Both switches can turn the light on and off. However if both switches are in the up position, the breaker trips. I've checked and rechecked the wires. They are correct per my Black & Decker Complete Guild to Home Wiring, and the first few pictures on google that came up. What could be the problem?

I have a project I want to try that will be a strip of LED lights along the stairwell. I want to switch them on and off with a beam. Like what causes a door to chime in a store. When the beam breaks, the lights go on. Then either with a timer or another beam break, the lights go off. I have a few criteria for the beam. I really on have access to one side of the stairs behind teh wall. So I am thinking some sort of laser that reflects. I want to put this inside the wall, so garage door things are out as they are too large. My question is where can I go to find some good information on stuff like these? Searching has either resulted in garage door or outdoor style beams or beams that are only good for up to 18"