Where can a buy a tool to open mine? I have 3 inch diameter cleanout plug with a female square slot insert that is 13/16 inches. I cannot locate a tool anywhere in HW stores or the Internet to fit this 13/16 inch square female slot.

Thanks.

If you Google 13/16" cleanout wrench you will find many available, Here is one.
http://www.ebay.com/itm/Camco-RV-397...pV5JAr&vxp=mtr

Open up your needle nose pliers and stick in slot.

Or this if you have one

If you go to plumbing stores that cater to the trade, they will have it in stock... Places like Ferguson Enterprises, Hirsh Pipe and Supply, etc

Milo