This paper proposes a three-phase integrated boost multilevel converter (TP-IBMC) based high power IPT system. Using a digitized modulation scheme, the proposed TP-IBMC can output a square wave voltage with variable amplitude to regulate the power transfer. The TP-IBMC includes three bridge arms, each of which contains a DC inductor along with multiple sub-modules to realize variable voltage levels. This paper also establishes a mathematical model of a three-phase IPT considering the mutual inductance between phases. Using this model a capacitor voltage balancing strategy for the TP-IBMC is discussed. A 6 sub-module primary-side controlled IPT system compliant with the SAE J2954 WPT3 standard is designed and simulated to verify that TP-IBMC can regulate output power through a digitised modulation scheme, under a range of coupling factor (i.e., from 0.14 to 0.31) and EV battery voltages (i.e., from 300V to 420V).