I'm just going by what Thomson said. That Merrifield will play some 3B and 1B. Of course that should be based on him being the hitter he was three years ago. If he's struggling/declining he can't necessarily outhit Sosa or Bohm, let alone out-defend them, and would have no business playing 1B (even if Clemens and Cave didn't either last year).
Miller's about the same as Merrifield I'd say, though he certainly wouldn't be as expensive. In addition to his bat fading not sure he can really field that well anymore either, though he's obviously coming back from a lost season/injury.
I actually think Sosa is fine if Bohm is injured. It would just make more sense to have a LHB to spell him once or twice a week on the regular. That player probably wouldn't be a good defender himself, or anyone you'd want playing 3B long-term (if Bohm was so ineffective as to lose the job, which seems unlikely).
Sosa was kind of bumpy last year at 3B but he should still be capable there defensively in the long run.