I had a root canal 3 months ago but tooth is still sensitive to hot. what could be the reason?

Hot sensitivity: A tooth should no longer be sensitive to temperature changes after a root canal. Some teeth will have canals that are very difficult to find, even by the best of dentists. Go back to your dentist and discuss your options. You may consider seeing a root canal specialist, most have surgical microscopes they can use to help locate these hidden canals.

Hard to say: A tooth with root canal therapy should have not sensation to thermal stimulus, either hot or cold. It might be an adjacent tooth. Consult with your dentist to have your other teeth evaluated.

RCT sensitivity: I will assume that the rct was completed. Properly treated root canal therapy teeth cannot have sensitivity to cold, hot or sweets. If the tooth is still sensitive, it's most likely another tooth. Patients have difficulty sometimes pinpointing exactly which tooth causes pain. See your dentist for an evaluation.