Hi, 3.5 cm mouth opening is mild form of OSMF, for which usually medical management is sufficient. You need to take antioxidants like multivitamins (SM Fibro, once daily for 3 months) and fresh fruits, vegetables.
Topical use of Triamcinolone (Kenacort 0.1%) can be used to reduce symptoms like burning sensation. If symptoms persist you need to visit a maxillofacial surgeon for further evaluation and treatment.
Next Steps
Quitting of habits like gutka chewing, smoking and consumption of alcohol.