Diagnosis
The safety reversing sensors are obstructed or misaligned. The three-beep pattern is a specific alert for this fault.
How to Fix It
Look at the two small sensor units on the garage door tracks, about 6 inches off the floor. Their indicator lights must be solid, not blinking. Ensure nothing is blocking the beam between them. If a light is out or blinking, check the wire connections at each sensor and at the motor unit. Loosen the sensor bracket, align it until both lights are solid, then retighten.
If That Didn't Work
Direct sunlight can flood the sensor's receiver lens. Wait for a different time of day or shade the sensor with your hand to test. If it works when shaded, you may need a sun shade kit (Genie part # 30840T).
Heads Up
Do not attempt to operate the door with a solid blinking light on either sensor. The door will not reverse if something breaks the beam, which is a serious safety hazard.