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

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.
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.