for a class 3 medical you are required to undergo the same tests as the class 1 i.e ecg, full audiology & opthamology in addition to the class 2 tests.
The only real difference between the class 1 & 3 medical is the validity period i.e. 2 years instead of 1 year for the class 1 and the fact that with a class 3 you can have some medical conditions that preclude a class 1 as long as they are managed.