3 gallons per puffer, your 3 are good how they are.
They like to overeat so don't overfeed them. I've heard about 2-3 bloodworms (or equivelent) per puffer, make sure to watch them all to make sure everyone's getting food.
Lots of plants and other decorations to break up their sight lines is a great idea, because otherwise they won't get along.
It's generally thought that the wearing down of the teeth isn't as important with dwarf puffers. Putting snails bigger than bite sized in the tank will cause the big snails to endure a slow painful death of getting nipped.
Good tankmates for DP's are otos and sometimes shrimp, it really depends on your puffers. I'm lucky and my DP's are pretty docile for the moment. Seeing how they've all reached sexual maturity I'm hoping they stay that way. I did however add a few more recently and these ones seem a little more aggressive, so I'm kind of on edge about whether or not the peace will be kept.
Ok after all that, enjoy your DP's. I saw them on the internet and thought they were cute. I got a few and fell in love. They're out all the time, they dart around and chase eachother, they love investigating every little thing, they're awesome. In fact I like them so much my next plan is to upgrade my tank to make more room for these little guys. Maybe add some otos and perhaps if there's room more shrimp.
By the way, dwarfpuffers.com is a great site.