Most schools have online application systems that allow you to submit up to five recommendations. (NOTE: You don't have to have
LORs submitted online, but it's an option that my professors preferred.) 3 is the minimum, but send 1-2 more if they add something substantial to your application. Don't send mediocre letters just to have a 4th
LOR... it won't help, and it may create doubts that wouldn't exist if you only sent 3.
All that you submit are official.