Stage 1 is a paint cleanser and shouldnt be used too often! Its like a deep cleanse that gets rid of the crap on the paint you cant see, I believe this contains some abrasives (mild). You dont need to use this too often. 3 coats is also a bit excessive for this level.
Stage 2 is a polish (much like Autoglym Super Resin), basically fills in any minor marks you have on the car, such as swirls etc.
Stage 3 is a carnuaba wax which tops it all off (you can layer this one a fair few times, say each time you wash the car, and it should look great)
Just Stages 2+3 every few months should be ok.
Get some applicator pads to apply the products with, and some microfibre cloths to for buffing off.
Ive never used this 3 stage product, thats just what I know about it!