At the beginning of October we got a new 12 week-old beagle, Copper. We brought him home to our other two dogs, Rusty (1.5 year-old bluetick) and Lucky (14 year-old beagle). At first everyone got along just fine, but one day when they were all outside Copper tried to play with Lucky. Lucky didn't want to play and got mad with Copper and started growling and snapping. Rusty then started to fight with Lucky because he wanted to protect Copper. This was about a week or two after we got Copper. Now, we can't let all three dogs be with each other because Lucky doesn't like copper. If copper didn't try to play with Lucky everything would be fine, but all copper wants to do is play with everybody. Copper is now about 6.5 months old and lucky still doesn't like him.
How can I train Copper to stop trying to play with Lucky, or train Lucky not to get mad at Copper, and train Rusty not to fight with Lucky when Lucky gets mad?
Also, Lucky was never socialized as a puppy so he doesn't get along with very many dogs.