I had root canal 2 years ago, 3 dentists looked at xray and say it's fine. problem is when it touches food etc it is so painful, what should i do?

Possible fracture: A root canal takes out the nerve and blood vessels of the tooth but the tooth is attached to your bone by ligaments or fibers which is still alive. If there is a fracture it may not be visible on an x Ray. Sometimes an endodontists microscope can see it. Also have your dentist check your bite or occlusion. If there is trauma causing your pain then your bite needs to be adjusted to even your bite

Second opinion: In many cases, seeking a second opinion is not only warranted, but necessary. Unless your disease is life threatening and requires emergency care, it is never a bad idea to seek a second opinion. Getting a second opinion helps you feel more confident about your diagnosis and treatment plan.

Have a clinical exam: A second opinion is a great idea. You do not mention how the tooth was restored. It might be a good idea to have a dentist rule out a fracture, which can definitely cause discomfort. You also do not mention whether the three dentists saw and examined you clinically or only looked at the x-ray.

See specialist: An X-ray is a 2 dimensional representation of a 3 dimensional object. It's a great diagnostic adjunct, but must be coiled with at least a clinical examination. Please see an Endodontist, a root canal specialist.

Endodontist: Find an Endodontist who has a CBCT unit to further evaluate the tooth and bone. This is a CT scan unit. With it we can often see things not visible clinically or with traditional x-ray imaging. Often there is a crack that does not (yet) show up. See Prosthodontist for tx options should you decide to have the tooth extracted. Get clinical exam.Good Luck.