Not an adventuregame, but if you play Baldur's Gate you are given a pair of Golden Pantaloons. If you then import that character in Baldur's Gate II and find the Silver Pantaloons and then import that character in the Baldur's Gate II expansion and find the Bronze Pantalettes, you can give all three to a gnome who construct a mech armor, a rocket launcher or a flamethrower for free. That's the only easter egg I know off that requires the combination of 3 games to unlock
