The PS-5 is basically a new version of the harmonist pedal with some other pitch shifting stuff thrown in.
The PS-3 & PS-2 are completely different. They are more useful for combining pitch and delay. You can get some very out-there effects by tweaking the feedback and pitch knobs while playing. The PS-3 has a few more modes than the PS-2 and also has the added bonus of control via expression pedal and 2 outputs.
The PS-3 is one of my fav Boss pedals just because it is so much fun to play with.
Well, they have delay which the ps-5 can't do, and it is a very nice sounding ambient delay. I only own a PS-3 and it is radically different than the PS-5. I would not use it for simple or intelligent pitch shifts (harmonizing), because I think it sounds kinda crappy in that department. It does, however have great delays, and a really cool synthy sounding reverse mode, similar, from what I understand to the famous "crystals" mode on eventide units (though obviously not $1000 quality). It is a very unique pitch shifter, and on my board I chose the ps-3 over the ps-5, because the PS-5 (apart from the harmonist mode) to me is rather gimmicky.