No, they don't puff, or they shouldn't puff. So hopefully you don't want them for that reason.
You could call Petsmart and see. They most likely won't give you any money or credit for them, but they might just take them for free.
I haven't heard of them eating freeze dried. You could try soaking them in tank water and then feeding it though.
Here is more about feeding them:
http://www.dwarfpuffers.com/care1.htm.
3 would be fine if you don't want more than that. The reason for getting more is to get a better male to female ratio. You have a better chance of getting more females when you get 5-6. Its best to keep them in ratios of 1m/2f, but it can be hard to sex them when they are really small.