Where do I send additional recs?
If Princeton has its say, the answer would be nowhere. PTon feels your 2 LORs are completely sufficient. However, anecdotally, it’s been said a 3rd compelling and DIFFERENT LOR could be acceptable. You’d send it to the admissions office of course.
Additional LORs, in general, are NOT wanted and only serve to water down an application. Act as you see fit.
@T26E4 My DS was recently accepted to Pton SCEA. He sent an additional (hard copy) letter of rec to the admissions office address on the Pton website. His letter of rec was not from a teacher, and he felt it truly added another dimension to his application. Obviously it didn’t hurt too much. 
So, @fifteenpapers if you feel that this LOR is truly different, then my opinion is that you should submit it.
I agree with squeakywheel. Both my children sent one additional recommendation and both were accepted. Don’t inundate the school, but one additional recommendation from someone who knows you well and has a different slant than previous recommenders could well be helpful.