Aggression issues, water quality issues.
You can sometimes get away with fish surviving, but it is a mixture of advanced fishkeeping and plain old luck. And of course, "surviving" is a heck of a lot different than thriving.
You may have gotten lucky with 3 puffers that got along, and had really good quality tap water and did massive water changes and maybe just got lucky and they didn't die right away. Other people might end up with 3 puffers that can't stand each other (you never know what their personalities are going to be), have water that isn't quite as healthy for the fish right out of the tap or just had fish that were already sick from the store or parasites or whatever...
We suggest what we think is best for the fish, if they are kept in captivity. Not what they can "survive" in. Fish are tough and can "survive" a lot of things, but if you are a good pet owner who has committed to the responsibility of caring for the life of a live animal, you do what you can to keep them comfortable and healthy.
