I will say this much. JD Martinez is the most likely to be successful in Fenway.
Martinez is largely a pull hitter (39%) with flyball tendecies (38%) who hits the ball hard (39%) very frequently when he makes contact.
Santana is more of a pull hitter (51%) who hits about as many flyballs (39%), but fewer hard hit balls (33%). Fenway might be tough on him when hitting lefty.
Hosmer is a ground ball machine (53%) who rarely hits the ball in the air (22%), and doesn't always hit the ball hard (33%). He might develop power, as he is about as old as Martinez was when JD did start hitting for power. and Hosmer has been surpressed by his home park, where his 119 OPS+ the past 3 yesr is still impressive and a sign he is developing somewhat as a hitter.
I'm not wild about JD Martinez' age, but if anyone thinks the Sox should have signed Encarnacio last yer, his baseball age is 4 years older. If you would have signed him 4 years ago, JD is a decent comparison.
The one big disadvantage is position. JD has never played anywhere but OF, where the Sox have 3 excellent players. Sure they could move him to first, but he might not want to go and might not be any good. And if he doesn't want to go to 1st, the Sox would have to break up a terrific defensive OF, likely by moving Bradley. They would still also need a 1B. Bradley to SF for Brandon Belt makes sense talent-wise, but Belt has a longer pricier contract and a very recent and alarming history of concussions.
Stanton would trump all these guys, but he won't be as easy to get. And I think a very unlikely acquisition at this point.
All of these are career numbers for these players, and certainly all could have developed somewhat in the past 3 years. But that is more math than I want to do right now...