All colleges – not just Harvard – appreciate and respect student’s that can follow directions. For Harvard, that means two (2) teacher recommendations (not 3 or 4 or 5 or 12).
The exception is if you have worked as a research assistant, then you should have your mentor upload a 3rd LoR. Or, if you have an afterschool/weekend job, then your boss should write a 3rd LoR that addresses your maturity and ability to relate to the public.
FWIW: You should have a conversation with your teachers and they should follow the Common Application directions and not send, but upload their LoR after you have invited them via their email address. See: https://appsupport.commonapp.org/link/portal/33011/33013/Article/797/How-do-I-assign-my-recommenders