With the popularization and application of microgrid projects, the multi-microgrids (MMGs) with three-phase / single-phase (TP/SP) architecture is formed in a plurality of adjacent microgrids with certain needs. Due to the complexity of the topology and the diversity of the power supply modes, the MMGs can operate in various modes. It is an urgent problem to realize the flexible seamless switching of the MMGs under different operation modes. In this study, the topology and the hierarchical control framework of TP/SP MMGs are illustrated first. The mode transition strategies considering the constraint of three-phase current unbalance for TP/SP MMGs are proposed, to effectively coordinate and control the distributed energy resources (DERs), energy storage systems (ESSs) and loads during the switching process. The simulation results verify the effectiveness and feasibility of the mode transition strategies.