considering the 295x is already techinically a crossfire, when you add a third it will be atri crossfire. It might actually reduce your performance. Plus if it does, you probably wouldn't even notice.
With a 295x2 being so powerful, if anything is holding back your gaming experience it would be the rest of your components.
It would not be worth adding a 290x to a system that has a 295x2 in it already.
Plus the power draw for the 295x2 is ridiculous as it is, getting a 290x would eat power like the cookie monster eats cookies.