would pretest for ophthalmology be too much for a 3rd year med clerkship?

Are you asking about studying "Pretest for Ophtho" during your 3rd year medicine clerkship? If so, I like the excitement and interest, but concentrate on doing the best you possibly can in medicine! If you have a patient with an interesting eye finding, by all means read about it, but don't forget about the importance of an honors grade in medicine.

would pretest for ophthalmology be too much for a 3rd year med clerkship?

yes. likely too much for a clerkship.

you should ask the clerkship director or chief resident about what test (if any) will be at the end of the rotation. then ask what the recommended reading would be.