Oral Cancer presents itself usually as oral ulcers of long duration. A non-healing ulcer of greater than 10-15 days needs utmost care, where clinical examination by a specialist and may be a biopsy becomes mandatory. With the history of 3 days, I doubt, it will be cancer but cannot rule it out without a clinical examination.

Please contact to your dentist.dentist properly examination of your oral ulcers.and give you suitable medicine.