Let's assume that Helen is telling the truth, then Helen is a fox, Mary is lying, then Mary is a badger and Darla is a squirrel. But Darla said that Mary was a squirrel, which contradicted the hypothesis, so Helen was lying.
From what Helen said is a lie, we can know that Helen is not a fox, so Helen is a badger. At this time, we can know that Mary is telling the truth, so Mary may be a fox or a squirrel.
At this time, assuming that Darla is lying, then Mary is not a squirrel but a fox, but at this time Darla can only be a squirrel, which contradicts the condition that squirrels only tell the truth, so Darla is telling the truth, which means that Mary is a squirrel, so Darla is a fox.