It doesn't.I may well be missing something, but how does beefing up Route 3 to interstate standards fix the geometry and density problems confronting the Braintree Split and the Southeast Expressway?
Fixing the Braintree Split requires a whole bunch of exit ramp reconfiguration, but I'm not convinced that it's an implausible project even if you assume that there's going to be zero ROW acquisition. Now, that's not to say life doesn't get a whole lot easier if you put ROW acquisition on the table - and fortunately, all the ROW you'd need is either commercial or parking lots anyway.
This is a real, REAL rough estimation and would need to be cleaned up a lot. I'll add signage in to it later. It's a real rough estimation of how you would go about inserting new ramps to facilitate South Shore mall traffic and keep 37 entry/exit traffic segregated from through traffic.