First, do an SDN search on letters of intent, letters of interest, and waitlists. You can find out alot of information that way.
With three schools, you'll need to know which is your first choice, whether or not submitting a letter is helpful (for some, where you've received an unmovable score, it's not) and then if you'd be benefitted by sending additional info...(new LORs, updated transcripts).
You can always call the school and ask what would be best.