I really don't think it matters. I had 2 OB (1 was the chair) and 2 family medicine letters. And one of those fam med letters was from someone who did a lot of OB and so she could evaluate me on that stuff.
But, I think the real point is: As long as you have very strong letters, you're good. If you have the choice between knowing you'll get OK letters from all OB's vs. awesome letters from 1 OB and 2 other specialties, go with the latter. You want your letters to scream how good you are!