The total would be about 5 ohm load, so I think you can probably get away with the 4 ohm setting, since it's a very slight mismatch. But PRS customer service is awesome, so they can probably tell you better by far than I can!
Why not just run the 212 extension cab by itself? The third speaker isn't really going to add very much, and the amp will be running at its rated impedance, which is probably a safer bet. All you have to do is unplug the built in speaker, and you're golden.
212s can be plenty loud.
If you need a bigger spread or more push, just use a 412. Then you can still run at a rated impedance without concern.
When on a tone quest, running three speakers seems to be a great way to help you triangulate where your desired tone is.
Why not just run the 212 extension cab by itself?