My 1099-div includes Box 3 (nondividend distribution) but nothing in boxes 1 or 2. My basis will go below 0, where do I report the capital gains?

My understanding is that if the basis goes below 0 with returns of capital, then the excess should be reported as capital gains.  Can I add information to boxes 1 and 2 to reflect this, or do I enter this elsewhere in turbotax?