[Grah the great]

Can't help much with a diagnosis, but since all three fish are acting the same you may want to consider increasing your water changes just in case your tank has some sort of toxin that's affecting the dwarf puffers (who need VERY clean water and are unexpectedly messy for their size).

 

[bsantucci]

Why is ammonia showing up in the water? Is your tank not fully cycled? I'd put my money on that, even as little as you have. As mentioned above they need immaculate water. Definitely do a change.

 

[AquaAurora]

Dwarf puffers do not tolerate in-tank cycling, they are extremely delicate fish. You should have a pre-cycled filter (done fish less.. ammonia style is the best imo) BEFORE you buy them. They're likely to die from ammonia poisoning.
Also what size is your tank?