A, B, and C own the reversion in fee. At the conclusion of the lease, the land will revert, and the necessity will end. This may also interfere with a claim of strict necessity in B, but the better answer is strict necessity until the end of the lease. The Statute of Limitations is 5 years. The dirt road has existed since the early nineties.
Nothing is said about mental intent.
C. Continuous for the period of the statute of limitations. This is clearly a major problem. Estoppel Veloper used the dirt road to construct the house on Lot B at a time when ABC were undivided. If the three lots are treated as a single unit, then the use of the dirt road was for the benefit of all three. But only Lot C is landlocked, and therefore it can hardly be said that Veloper relied on access to the dirt road in building the house.