The increase in the generation capacity of the variable photovoltaic units introduces new challenges to the operation of the unbalanced three-phase distribution networks. In this paper, a two-stage optimization problem is formulated to identify the feasible dispatch margins of photovoltaic generation considering the distribution network operation constraints. The proposed problem is solved using the column-and-constraint generation approach. The distribution network constraints are formulated as second-order cone constraints. The uncertainty in the forecasted demand and maximum photovoltaic generation as well as the unbalanced operation of the distribution network is considered in the proposed approach. The dispatch margins of photovoltaic generation are quantified considering the worst-case realization of demand in the distribution network. The impacts of energy storage and the ramping limits of the dispatchable generation resources on the dispatch margins of photovoltaic generation are addressed in this network. The dispatch margins of photovoltaic generation are quantified in the modified IEEE 13-bus system. It is shown that enforcing the ramping rates for the dispatchable units will increase the lower dispatch margins of photovoltaic generation, and leveraging energy storage increases the difference between the lower and upper photovoltaic dispatch margins.