I can't find anything about Cox's being self-fertile. It's temperamental in our climate, unless you're in the south. Jonagold is a triploid, but Cox's should pollinate it. You still need a pollinator for the Cox's, but there's a good chance there will be a tree within range. If you want to plant a third tree, James Grieve or Worcester Pearmain are easily available, and good apples.