I have a Class 3 FAA and I instruct at a Part 141 school. In fact a FAA Inspector told me never to get a medical of a class higher than you needed. A FAA CFI doesn't even need a medical to instruct as long as the student has one and is qualified on the aircraft. Nonetheless one can instruct ab initio with a class 3. Trust me I know! Oh yes and one can be a Pilot Examiner too with a Class 3.