Can 3 inch magnum shells be used in an old shotgun?

First of all you would have to determine if your gun will accept 3" shells.Second,if your gun is very old it may not be adviseable to fire it at all.Have a qualified gunsmith check it out.  
In a word, NO. When you say very old and worn out you don't want to add the word MAGNUM. DO NOT USE 3" SHELLS.