I don't think anyone expects Martin to be very good in the last couple years of his deal. The one thing he has going for him is that pitch framing doesn't really decline and he will still be decent defensively. And for a guy who seems to really understand bio analytics if there is a case for a guy to age gracefully it is probably him.
As for your previous point the Red Sox could very easily get all 3, but it would be a huge cost. Any team could get all 3 if they overpay enough but teams know that it would kill the team's payroll.