three lights in triangle shape bright flashing and incredible movement

Fall of 1974, I was doing school homework in my bedroom (across the hall from my parents room).  My mother always looked at the sky before closing her bedroom curtains at night each evening. On this night, she called me to look at something in the sky that she thought was strange. I crossed the hall and looked out her window and saw three lights, triangular shape with one light at the top and two on the bottom, drifting slowly across the sky. I told her it was a jet, but she said "no, they were doing something strange a moment ago".  A minute later, the lights stopped moving and then began to flash (white flash) one at a time with the brightness of a flashlight right in your face. The lights began to flash faster and then they suddenly moved at three different angles at an incredible rate for which I know of no craft that could do this. Then the lights suddenly moved straight up and disappeared.
Date is approximate.  PD