Nickel/alumina/epoxy three-phase composites consisting of nickel particles dispersed in alumina/epoxy composite matrix were prepared using a facile wet impregnation process. The influences of in situ formed alumina particles (i-Al2O3) and epoxy on the dielectric properties of the composites were investigated in detail. For the composites with nickel contents far below the percolation threshold, the main electrical conduction mechanism is hopping conduction, and the main loss comes from polarizations. Therefore, the formation of alumina and epoxy result in higher permittivity but high loss due to the enhanced interfacial polarization. For the composites with nickel contents below but near the percolation threshold, the main electrical conduction mechanism is metal-like conduction and the main loss comes from eddy currents under high frequency electric field. Accordingly, the i-Al2O3 and filled epoxy between nickel particles could bring both high permittivity and low loss. This paper experimentally shows how the characteristics of the interface between different phases influence the dielectric performances of composites. And the impregnation process is an effective strategy to obtain percolative metal/ceramic/polymer three-phase composites with ultralow metal content, high permittivity and low loss.