I thought RPi 3 has the same size and mounting hole locations with RPi 2 (and RPi 1 A+/B+). If so, it should fit the current version of OSPi (1.4+).
My ideal RPi (which I wish would exist in the future) is basically RPi zero with a standard size USB port, and with the 2×20 pin headers pre-soldered. If this exists at $10 to $15 price range, it would be perfect for OSPi.