You'd be insane to go to Boston in 3 hours. As a MR you're not bringing bags, it'll take you 25 minutes from seatbelt sign off to a taxi cab. It'll take you 15 or so minutes to get downtown. You would need to leave by 4:25 to get back to Logan for your turnaround. So at most you would have 45 minutes in Downtown Boston. Good luck with that if you choose to do so, and have a nice MR