I didn’t apply ophtho so take this with a grain of salt. Get to know another ophtho attending who can write you a letter. You have ~9 months to do so. If you can’t get a 3rd letter at your home institution, do an away rotation. In my opinion, if somebody can’t form a professional relationship with 3 faculty in order to get letters in the field, I would wonder why they couldn’t get enough letters and must conclude they must a) not be very interested in the field, b) not be on top of their ****, or c) be difficult to work with.
If you are applying to a small field like ophtho, you should be getting to know people in your home department. This is your RESIDENCY application - do it once, do it right. Put in the effort to get that 3rd letter so you won’t have any regrets later.