3 month old becomes fussy in evening, crying out suddenly as if in pain. Drooling alot and putting hand in mouth. Can he be teething?

Early. But yes, could be. See your Pediatrician to make sure it isn't something else. Also, in nine months give your child a wonderful 1st b'day present. Take him/her to a pediatric dental specialist, a Pedodontist, for initial exam, oral hygiene instructions, dentofacial growth & development evaluation, decay prevention advice, and so much more.