Where do nondividend distributions show on my tax return?

Nondividend distributions shown in Box 3 of a 1099-DIV don't go anywhere on your tax return. They affect the cost basis of the stock that gave the distribution.

So you reduce the cost basis of the stock by the amount of the nondividend distribution and keep that in your records. There is nothing to report on your current tax return if you didn't sell any of that stock.