Carlos Santana and Lonnie Chisenhall are both competing for the Tribe's third-base job, but to understand why last year's starting catcher and third baseman are now in competition with one another, you've got to go back a bit, to when Yan Gomes didn't play for Brazil in last year's World Baseball Classic, because that's the main reason all three of those players are in their current position. The beginning of this Terry Francona video tells the story concisely:
When Lou Marson was run over by Desmond Jennings (in a play that may be banned in 2014) in the first week of last season, Gomes was added to the roster. And as the season went on, with Gomes impressive behind the plate and a weapon in the lineup, Francona had a difficult situation to make. He could have kept Carlos Santana as the everyday catcher, and I don't think many would have blamed him for it, but not only did he make Gomes the starting catcher during a pennant race, he got Santana to buy into what had to seem like a demotion. For the last couple months of the season, Santana played more at first base and DH than at catcher. All the while he continued to be one of the best hitters in league, and got some MVP votes after the season.
But that decision during last season created another tricky decision after it: what the heck do you do with Carlos Santana? The easy decision would be to make him a full-time DH, and have him just concentrate on hitting. But again Francona wanted to try something unconventional, and that was to see if Santana could play third base, his position early in his professional career. So during the Winter Leagues in the Dominican Republic Santana played third base, and played it well enough so that it's a very real possibility that Santana will play there at least part-time for the Indians this season.
Santana has completely bought into the idea of moving to the hot corner. In this interview he mentions the good effects it will have on his bat, with Francona later talking about the possibility:
Of course if the transition works out, then Francona has yet another big decision to make, and that is what to do with Lonnie Chisenhall. But as Francona mentioned at the end of the piece, he has a responsibility to put the best lineup he can onto the field, and if Santana can become at least a fair defender at third, then that lineup won't include Chisenhall. Santana is one of the best hitters, regardless of position, in the majors, while Chisenhall to this point hasn't yet shown at the major league level the hitting prowess he has shown in the minors. But it's worth finding out whether Chisenhall can do that, though that would make constructing the 25-man roster rather difficult.
With Francona preferring an 8-man bullpen, that places a premium on versatility for the bench players. Santana could play third base and back up at catcher, but that would be seem to be only a temporary solution. Eventually, if Carlos proves he can play acceptable defense at the hot corner, he's going to be a full-time third baseman. And when that time comes, that means Lonnie Chisenhall will no longer have a position.
All these plans may not survive the first week of the season though, as we saw last year. Because last year the plan was that Yan Gomes was going to AAA and that Carlos Santana would be the full-time catcher.