Exploring and utilizing the reactive power support capabilities of electric vehicle charging stations can optimize distribution network operation and help mitigate voltage limit violations caused by three-phase unbalances in load, line parameters, and new energy output. Additionally, it can reduce voltage regulation costs for the grid. Hence, this paper proposes a voltage regulation ancillary service model for electric vehicle charging stations while considering three-phase unbalance. First, based on an analysis of the market mechanism for voltage regulation ancillary services and the charging behavior of electric vehicles, the daily load curves of slow and fast charging stations are predicted separately, and their respective reactive power support capabilities are evaluated. Second, a multi-time-scale day-ahead and intra-day optimization framework for distribution networks is established. The day-ahead optimization model aims to minimize the voltage regulation costs of the distribution network, determining the on-load tap changer positions and the scheduling of capacitor banks. The intra-day rolling optimization focuses on minimizing network losses and compensation costs of charging stations by adjusting their reactive power participation in voltage regulation ancillary services. Finally, simulations on the IEEE33-bus system are conducted and the results show that the proposed model can effectively reduce both three-phase voltage unbalance and voltage regulation costs in the distribution network