Super-Strong 3-Way Miter
Combination of hand and power tools makes it surprisingly straightforward
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