You can not mix ordinary switches with PCS unless all you want is a way to completely deactive your PCS switches.

You really need to nail down what wires are going where. Shut off the power and get out your multimeter.

With a 12-3 running between all boxes you can at least go with the in-wall keypad transmitter in the last location. This will give you X10 control of that circuit plus you can use it to activate up to seven scenes, depending on which keypad you choose. Unless the last switch location is chained off a different box, I don't see how you can put a slave there.

If I were doing it, I'd go with keypads any of the slave locations where scene control would be convenient.