Error 3 can be caused by a few different things:
Blocked filters: have you cleaned your filters lately?
Air lock: Have you just filled up your spa? You might have an air lock that can be fixed simply by loosening the barrel union at the top of the the circulation pump to bleed the air.
Faulty circulation pump: Worn or bad bearings or an electrical issue can stop the pump or make it run less efficiently resulting in low water flow and an Error 3.
If you still can't remedy the situation, please call our Customer Service Hub.