How do I enter a 2017 1099-DIV with a non-dividend distribution (Box 3). I was able to open the Form 1099-DIV worksheet and enter it in Box 3 there. Is this correct?

You just enter the the 1099-Div, in the normal 1099-Div place. At the 1099-Div screen, you will have to check the box "My form has info in more than just these boxes" to get box 3, to make the entry.

A non dividend distribution is a "return of capital" and is not taxable. So, Turbotax will do nothing with it. It is for your records. You will reduce your cost basis in the security, by that amount, for when you sell it in the future.