Value of my autographed baseball?

I have a baseball autographed by HOFers Gary Carter and Bob Feller and plan to get Bob Gibson in a couple weeks. I want to make it a HOF baseball full of HOFers. I don't plan on selling it, but am curious if having three players on different teams and from different eras devalue the baseball then if it were just to have Carter's autograph for example. Also how much would a certificate of authenticity add to the value?