Intriguing question. Be tougher now, with four-team divisions...
So far the winner is Jack Concannon, Bears/Lions/Packers back in the 60s/70s. Ah, here's an even better one -- Ralph Gugliemi, who played for the Giants, Redskins, Eagles, and Cardinals when they were all in the old six-team NFL Eastern division. That one's gonna be tough to beat.