Cats will sometimes inexplicably just stop eating whatever food they have been eating for months, years, etc. I do think it is odd that all of yours decided at once to boycott their food. Try a new package of their usual food. If that doesn't work, try another brand. If that doesn't work, get them to the vet ASAP. Cats can go into kidney failure quick. Not to scare you, just so you know that it can happen in just a few days.

Any stress in their environment?