You sound Canadian saying "from North America". It's fine if you have 3 letters, all from retina or all from oculoplastics.
Mediocre letters may not be fatal, but they don't help.
Which letter would you prefer?
1. Chair of Bascom Palmer or Wills..."Chris was an good student. He occasionally knew the answer to questions but often didn't. He came to work on time. He attended all 30 days of the rotation."
2. Assistant Professor A. Smith of University of Ordinary School of Medicine...
"outstanding medical student...one of the most knowledgable students to have rotated...participated in the wet lab, demonstrated excellent fine motor skills...
2a. (one of my letters referred to me as something like having the potential to create a legacy and enhancing their reputation. I don't remember how it was put or even the thoughts conveyed, but it sounded pretty impressive. My letter was from a sub-specialty chief but he was not nationally famous)
