Runner,vegan and always been thin.recently started martial arts,my appetite increases, gained 3 kilo.can't stop eating,i'm worried. normal?

Not normal but...: I would not call this "normal" but it may not be a problem. It may be unrelated to starting martial arts but martial arts can regulate the flow of energy through acupuncture meridians which might increase your appetite and cause weight gain if you are underweight. Hopefully this is a temporary rebalancing but if this persists see a doctor. It also may be reasonable to consult an acupuncturist.