PS-3 replaced obsolete tech in PS-2. Just like PS-2 3V replaced the very early and first PS-2 5V boards.
Basically PS-3 gives more memory for sound files, more features, LED lights versus Bulbs, No battery, DCS and DCC capability, Ability to detect DCS on any rail when 2R-3R engine, and some other stuff on the tech side.
It is the last from MTH and what is supported the easiest. G