If you have had an ulcer for 3 days and are experiencing pain, it is recommended to avoid hard, sharp, salty, acidic, crunchy, or difficult-to-chew foods. It is also advised to clean your teeth and gums gently, use a saltwater or hydrogen peroxide mouth rinse, apply ice to the affected area, and consider using an over-the-counter oral paste to prevent friction. If the pain persists after 14 days, the ulcer grows or new ones appear, or if you experience other symptoms like fever or difficulty swallowing, it is best to seek medical advice.