I assume one of the controls adjusted the seat back angle. Normal Boxsters don't have that feature and I doubt the S model does either. There is a manual adjustment but since the seats are very close to the bulkhead, unless you're 5 feet tall, there is not a lot of backward slant available.
So, hook up the two that work the seat bottom and manually set the seat back angle and leave it.
I'm sure there's a hack by tapping one of the hot wires, running it through the switch and grounding it to make the 3rd switch functional.
I don't know any of this for sure. However, the seat pressure sensor should be the same on both. Same with heated seats if that was optioned. None of that fancy stuff on our 2000 Boxster N.
And then there's
this 
