Beltre and Martinez both seem like the more difficult players to replace. A switch-hitting catcher who can also play 1B, and a power-hitting, gold glove 3B. I don't see any obvious C or 3B candidates over the next 2 years, either through development or in FA. Projecting 3 years ahead I can imagine a few prospects who could fill those positions, but over the short-term it is hazy.
Ortiz is a different story. His bat is valuable but he locks up a roster spot. This is a franchise that likes to maxamize its resources and their roster flexability has been hindered by having one of the only pure DH's in the league. They have some important aging veterans (Youkilis and Drew) who would benefit from having regular rests at DH. Having Ortiz makes that more difficult.
I bet Ortiz gets offered what they want to pay him (2 yrs/$14m total?) If he leaves they open a spot and are assured of not getting into a bad contract.
The Sox could offer Victor Martinez 4 years, $40m and he might be happy with that. It would make him the third highest paid catcher in baseball behind Mauer and Posada and give him some security for a few years. Between C, 1B and DH they could afford to move him around and could even end his role as catcher altogether if they are able to upgrade at that position. Martinez has a .842 OPS since 2005. He's clearly a good MLB hitter and would be valuable as a DH and 1B if he got a lot worse defensively. He would only be 35 when the contract ended.
Beltre's spike in offensive production concerns me, but he's a 3-5 WAR player every year because of his defense. He could command $10m per-year too.
All in all (chance of resigning):
Martinez (50%)
Beltre (20%)
Ortiz (20%)