Abstract: Three-phase magnetic cores for magnetic induction devices (e.g., transformers, coils, chokes), and methods for manufacturing them, are disclosed. The magnetic cores are generally constructed from three generally rectangular magnetic core frames having a stair-stepped configuration extending along side portions of the frames. The frames are arranged to form a triangular prism structure such that side portions of locally adjacent frames are uniformly engaged to form three core legs over which coils of a three-phase magnetic induction device may be placed.