I have what looks like a canker sore on my gum. it has been there for 3+ wks. it doesn't hurt.

Not a canker sore: See your dentist, unless your immune system is seriously compromised, canker sores typically resolve in less than 7 days. See your dentist for an evaluation to determine your exact problem.

Gum lesion: Canker sores are caused by viruses and are usually painful. They can last anywhere from 7-10 days and usually resolve on their own. I would be concerned about a gum lesion that has persisted 3 weeks and is not sore, especially if you are a smoker or use smokeless tobacco. Have your doctor look at this lesion to determine exactly what it is.

Not a canker sore: Canker sores don't appear on the gums, they are usually on the soft tissue in the mouth or at the base of the gums, they usually disappear in one or 2weeks and they are painful, If it not going away, it is safe to consult a dentist.

Oral lesions: It may be any one of a number of things and it should be examined and probably biopsied to rule out oral cancer. See an Oral Surgeon as soon as possible.

Immediate evaluation: What you are describing is not a canker sore. Important that you see Dentist or preferably an Oral Surgeon for an examination, possible biopsy, differential diagnosis, and treatment.