Clarrification

So i should loosen the tensioner and turn counterclockwise 3 teeth then retighten? If I apply tension on the belt will it damage the belt or even come loose again, can it be hard on the valves?

The damage may already be done. You won't know until you replace the belt and go through the installation steps. I still wonder why the tensioner moved and the belt came loose. Did you follow all the steps and did you torque the bolt.