I have a single 1099-MISC with income in boxes 2, 3 and 7. When I enter, TurboTax says "You cannot enter income on mulitple boxes on the same Form 1099-MISC". Now what?

You will have to enter three (3) Form 1099-MISCs to report the information on the one (1) 1099-MISC you received - one 1099-MISC for each box of reported income (2, 3 and 7).

Why do you have to do this? 

Each type of income on a Form 1099-MISC requires a different set of interview questions and can generate different tax forms (e.g. Box 1, Rents, generates a Schedule E, Box 7, Non-employee compensation, can generate a Schedule C). 

Since most 1099-MISCs only report one type of income, this is not usually a problem. When they do report more than one type of income, the program needs to enter each type on a separate 1099-MISC, so each will generate the correct set of questions and tax forms