This miter joint has been in use since the Ming dynasty, and it may seem daunting at first. But Andrew Hunter lays out all the steps to create the three-way miter, starting with accurate layout, moving on to some slick hand-tool and machine tips will get rid of the unwanted material and leave you with perfect tenons and miters, and moving on to shaping.

From Fine Woodworking #227

Online Extra: SIM: Three-Way Miter Joint