Rotating the use of the DH is the most direct answer to how two players at the same position can coexist in the same lineup. Thought that was clear, but to be specific…
If Nunez and Walker are on the same team, they split time between third and DH until one stands out enough to be promoted ahead (or one struggles so badly, he must be demoted).
Mendoza is already a utility guy so he can play other positions most of the time. Further, he should be a lower priority than Montero, but he can play third when Montero DHs.
My bottom line remains the same. None of this is serious enough to move top 15 prospects in the system to lesser positions.