Pull your alternator and have it tested and check your grounds. My car was doing the same thing. Usually the Christmas tree is the celebration of a dying alt.
The sensors are designed to measure the difference in voltage going to and from the sensors. And when the voltage on the way back drops below a set level, it turns the lights on. When your alternator is not working right, it will either not send out enough, or send out too much.. Which causes the lights to come one due to low voltage. All three at once is a good clue.
But what do I know, I just work on planes.