Can this be used over a 3/4" plywood subfloor? If so, what would be the best underlayment for it?

Hello Charles and thank you for your inquiry. According to the installation guidelines, adding an additional 1/4" underlayment grade plywood on top of your existing 3/4" subfloor would be required to meet the requirements for installing a rigid core vinyl floor. The underlayment is also attached, so no need for anything additional. We would definitely recommend reviewing the Warranty and Installation Manual (found in each carton or on the product page of Lowe's website) prior to your install. Thanks again and all the best with your flooring project.