Is it a psychiatrist or pediatrician or psychologist who sees kids with behavior and learning disabilities?

Both: Both may see such a child. Sometimes kids need medication and that can only be provided by a psychiatrist. A psychologist cannot prescribe medications.

All three: Start perhaps with your pediatrician. He will then help determine what course should be taken and which is best.

All + neurologist: Start with pediatrician. Next would be a referral to a neuropsychologist for testing. With the results, next stop might be neurologist,behavioral psychologist, educational specialist or psychiatrist, depending on the result of testing