Assuming they are ALL "strong" and the date the application is submitted is unchanged...essentially all else equal....is it better to show your "versatility" with a letter from another field or three form ophtho would be best?
I actually had the same question you did not too long ago. I was told by both the program director at my school, and another "big wig" Ophthalmologist that it is an absolute must to have one letter from a core rotation (e.g. Medicine, Surgery, Peds, etc). I had planned on having 3 Ophtho letters too; I used 2 Ophtho and 1 OB/GYN, and will send the third Ophtho as a supplemental. Hope this helps.