What is the value of a Mickey Mantle Whitey Ford Bob Feller signed baseball?

Enough to make it worth your while to get it authenticated. There's a huge gulf between what people are willing to pay for a signed ball that looks real and one that a professional authenticator has given the OK to. Bob Feller and Whitey Ford signed balls are listed around $50 each, individually- (they are both still alive, and that affects market value/supply/demand on these things) but will generally go for at least $100 on something like Ebay. Mickey Mantle is hugely popular among aged 50-ish collectors and is dead besides, and all his stuff sells for three times that of his contemporaries. A signed Mantle ball lists between $300-400. All three sigs on the same ball, you might be looking at $700-$800, maybe more depending on condition, clarity of signatures, whether your dog got ahold of it, etc. The most important factor is whether they are authenticated by an expert--even if you were standing right there at some Yankees/Indians game in 1956 & watched all three sign it, people will pay much, much more for something if they don't have that niggling question about authenticity haunting the back of their mind. You might want to check out some of the Mantle items on Ebay and see just how crazy-expensive some of them are.