Odd, usually the pressure gauge screws directly into the RO membrane housing. I would think placing it after the 3rd stage would not tell you anything useful. Placing it before the RO membrane would be the only place I would think it would be useful because you want to know the pressure of the water coming into the RO membrane.. placing the gauge after the RO membrane isn't going to tell you the information you need. Most RO Filters go 1st Stage=Prefilter1, 2nd Stage=Prefilter2, then RO Membrane, then 3rd stage which is usually the DI resin stage.