@ChargerFans
"Extra recommendations. If you feel an extra recommendation would show an important additional side of you not already covered, you may send in an additional letter of recommendation. This in general would not need to be on an MIT recommendation form from the application; a separate sheet of paper is most common. In general, third recommendations from a teachers do not provide much additional insight; the most helpful supplemental recommendations come from people who know you well outside the classroom. Some helpful extra recommendations I've seen have come from research mentors, youth group leaders, coaches, and bosses. If you decide a third recommendation is necessary, have your recommender send this via US Mail to MIT Admissions, and be sure your full name and date of birth are included." - Matt McGann MIT Admissions Director
MIT Admissions | Blog Entry: "Supplemental Materials"
I have two extra recs from a Ph.D researcher and a econ Professor who I both worked for in two internships I had.