Is it possible for an 18 year old who smoked for 3 years to get mouth cancer?

Yes: There is no 'safe' length of time to smoke before the risk of cancer kicks in. Also, cancers often have many contributing causes--so smoking can be just part of the picture. Your best bet is to quit today. if you have mouth symptoms and you're worried it could be cancer, then see your doctor and discuss this. Your doc might recommend further evaluation, treatment, or other expert opinion.

Mouth cancer: Cancer can affect even young people. I think it will take more years of smoking to get it. If you have a lesion or worried about something in your mouth, see your doctor or your dentist who could check your mouth and biopsy any lesion if there is one.

Yes: Yes. Stop now. See oral surgeon or ENT doc if you have any of these symptoms: Persistent mouth sore, persistent mouth pain, a lump or thickening in the cheek, a white/red patch on the gums, tongue, tonsil, or lining of the mouth, a sore throat, hoarseness or feeling that something is caught in the throat that does not go away, difficulty swallowing/chewing.