a 30l tank is nowhere near big enough for 3 dwarf puffers, they may be tiny but they are so territorial that they need their own defined territories. The normal size is 20l per dwarf puffer minimum. Unless you have clearly defined your tank into 3rds and they stay in their territory that explains why one has puffed up, if the others have attacked it trying to get territories. If i were you, i would get the one that is looking ill into a breeding trap in the tank right away. Make sure the others can't get to it and each has his own space.
When i lived with someone who had dwarf puffers the only way that she could get them to eat was bloodworm. Are you varying their diet?
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