The 3", 5", and 6" are priced each and sold individually.
There are dozens and dozens, if not hundreds of choices of foam pads on the market. There are different colors, densities, shapes, sizes, surface types, etc. So which one do we buy? I choose these RUPES pads for a few reasons.
These have been designed and tested specifically to be used with long throw polishers, so they can withstand the heat and lateral forces that beat up the structural integrity of the foam.
I've been using these since RUPES first launched them and now the color codes are ingrained in my brain: Blue - Heavy Polishing, Green - Medium, Yellow - 1 Step or Finishing, White - Finishing.
The Yellow pad is probably the most used pad in my arsenal. I often use it with something like Sonax Perfect Finish to polish using a single step. You may also find that since this is a bit stiffer than the White pad, on certain paints you'll get a better finishing result.
As much as I like complete sets, I only find the Yellow and White pads necessary. Combine those two and a bunch of Meguiar's Microfiber Cutting Pads, and you should be able to tackle almost any situation. Sizing is a bit confusing, but it works like this.
1" and 2" Pads = Fit 1" or 2" backing plate and have 1.5" and 2.25" face.
Use with Nano
3" Pad = Fits 3" backing plate and has a 4" face.
Use with LHR75E (Mini).
5" Pad = Fits 5" backing plate and has a 6" face.
Use with LHR15ES, MKII, LHR12E (Duetto).
6" Pad = Fits 6" backing plate and has a 7" face.
Use with LHR21ES, MKII.
These pads are designed to be used with dual action random orbitals, specifically long throw machines. They aren't recommended for rotary or forced rotation.