Small hard bump on roof of mouth that gets softer and harder as day goes on. Have had it for three months, not painful more so annoying. Mouth cancer?

Cyst. It could be a mucocele but since you have had it for 3 months, i would recommend that you see your primary care or ENT specialist.
Exam. Statistically unlikely but possible. See oral surgeon or ENT doc for assessment.
See Your Dentist. It sounds more like a possible abscess or infection than cancer though that cannot be rule out without examining it. If infection, it could be a tooth or your sinuses either of which can explain why it is soft then hard but not painful - it is draining some where which makes it softer then pools overnight. You've waited too long to not get it is taken care of!