Mouth ulcers lasting for 3 months are not normal and need proper evaluation, especially since you also have a loose tooth with occasional pus, which suggests a chronic dental infection. The ulcer on the opposite side of your tongue and cheek may be due to repeated irritation (friction) or infection, but long-standing ulcers should always be checked carefully. The first and most important step is to visit a dentist or oral specialist to treat the loose, infected tooth; this may require cleaning (scaling), root canal treatment, or removal if the tooth cannot be saved, as the infection can delay healing of ulcers. For the ulcer itself, doctors may prescribe medicated gels for pain relief, antiseptic mouthwashes, and sometimes vitamin supplements if a deficiency is suspected. Avoid spicy, acidic, or very hot foods, and maintain good oral hygiene. If the ulcer does not heal within 2-3 weeks after proper dental treatment, further tests may be needed to rule out other causes. Early treatment of the dental problem usually helps the ulcer heal completely.