Chief, better replace it immediately as the swelling is a bad sign of the battery condition. I'm not threatening, but due to experience, several of the swelling batteries which I saw, ended up in exploding, I'm not saying it will happen to you but better be cautious than not . 3 years of service is already a "milestone!". Good luck chief!
Sir, I think you should replace it immediately. I think you shouldn't wait until the battery dies, because if ever an emergency happens ( knock on wood ) we don't know if your old battery will still bring you to the place where you should need to go. Or if you want to use your batter until the end, better bring a new one and put it in your trunk.