The 303 usually has a woven dacron mainsail with a boltrope (3 strand dacron rope in a sleeve at the luff).
Most sailors dont 'raise' such sails properly .... you need to raise and THEN apply additional strain to the luff to additionally stretch-out that boltrope so that the sail can take its 'designed shape'. If you dont do this the poor shape will cause a LOT of 'weather helm'.
Next time out, raise the main and then apply additional halyard tension by 1" for every 11 ft. of luff length (about an extra 3" for a P303).
Then, Put the boat onto a hard beat at 12-15kts and let go of the wheel. If the boat keeps a straight course or falls off to leeward ... then slightly ease the halyard until the boat 'slowly' heads up to weather. If the boat 'heads up' aggressively, add more main halyard tension and/or THEN consider to take some rake out of the mast.
A boat that is dragging its rudder sideways through the water isnt going to 'point' (may be skidding off to leeward) and is going to be 'cranky', etc.
The 303 has DEEP bilges and not much relative exposed keel surface area. If you heel a lot .... that keel is going to start to slip to leeward. Usually better to keep such a boat 'standing more upright' when beating. Watch how the wake is coming off the stern .... shouldnt be more than about 5° off from the boats centerline.