I have a 3 cm tumor in my sinus. they think it's due because of my deviated septum. what are the chances it could be cancerous?

Unlikely: Unless you know the features of this tumor in sinus(?Polyp) is hard to be sure. Your ENT doctor can advise you further. If there is any anxiety about its nature, a repeat x-ray of the sinus in 3 months can be more assuring that it is likely to be benign. Cancer of the sinuses is an uncommon cancer but it does occur. Usually it will produce blood stained nasal discharge and some local(facial pain).

Sinus lesion: Sinus rumors are quite rare. The most common lesion within the sinus would be a simple mucous filled cyst, which requires no treatment unless it is problematic. Studies can be done to help determine what the lesion is. I doubt that a deviated septum would cause any lesion within the sinus, however.

You need a biopsy: a septal deviation does not cause tumor growth. Nonetheless, you need a biopsy.