I occasionally get girls who fail to thrive, on occasion either because they didn't get enough food before cupping by the breeder or because they were cupped too young they sort-of starve to death.
I'm treating my sorority with kanamycin over the next couple days so I bought a "failure to thrive" candidate from walmart to see what happens.
Sometimes it could be something on the hands of people who handle the cups, something on your hands or many other kinds of pollution sources.
I caught a woman spurting purfume at the ferret bin in petco last week, it aerates and lands as an oil on the water in aquariums.
Not blaming you, the three day thing is something I commonly see, but take a look at the cleanliness of your care, air-flow over tanks, exposure to dust and fumes or cats.