I only have an amount in Box 3 of my 1099-DIV. What should I do?

Non-dividend distributions shown in Box 3 of Form 1099-DIV don't go anywhere on your tax return. They affect the cost basis of the stock that gave the distribution. In your personal records, reduce the cost basis of the stock by the amount of the non-dividend distribution to keep accurate accounting of your stock basis. There is nothing to report on your current tax return if you did not sell any of that stock.