The third one is definitely worth it. Some of the levels are slightly more linear in the way you're encouraged to beat them (although definitely still work as sand boxes) since it was obviously an audition for the Bond game they're now making, but that also let them do some really cool remixing of the usual Hitman formula. The Knives Out-meets-the-illuninati detective mystery and the Hunter-becomes-the-hunted-and-back-again Berlin rave levels come instantly to mind. Also, just in general, I don't think the vibes have ever been stronger than they are in that third game.