Where do I put non-taxable income for box 3 of 1099-DIV
There is no data entry field on screen DIV for nontaxable Nondividend Distributions on line 3 of a 1099-DIV. An entry is not required.
Help (press F1) for line 3 of the DIV screen refers you to Pub. 550, Investment Income and Expenses, for an explanation of Nondividend Distributions.
See also Pub. 17 for more information on Nondividend Distributions.