This one's easy -
But seriously, beyond Wright, they have a few players who would work as reserve third basemen, such as Daniel Murphy (came up as a 3B), Justin Turner, Nick Evans, etc.
They have some minor leaguers, too, who could provide some production at third base. Zach Lutz is likely to start the season as Buffalo's Third Baseman, after splitting .287/.380/.571 between multiple levels, mostly in Binghamton, but with 5 games in Buffalo.
However, the main prospect for the Mets at 3B is Aderlin Rodriguez. Rodriguez played in Rookie Ball and Low-A Savannah last year, and ended up with a split of .300/.350/.532. Rodriguez projects to have among the most potential power among current Mets prospects, although he may be forced to move to First Base or a Corner Outfield spot in order to stick in the Majors. Overall, though, he's the one to keep an eye on going forward.