You don't need to replace any of your DVDs. They'll play just fine in your Playstation3. And it'll even upscale them to 1080p resolution (if you have an HDTV). I only plan to repurchase a few marque titles (Star Wars, LOTR, etc.).
As far as multiple consoles go, your TV should have multiple inputs, depending on how new it is. So you can hook your TV signal into Video1, Wii into Video2, your PS3 into Video3. Or if you have a receiver hooked up, you can do the same thing.