Short time voice loss could mean that they have an upper respiratory infection. Viral illnesses are also a possibility. Keep in mind that both are highly contagious. If this is the case, your cat should recover with time. Keep an eye on the secretions from the nose or eyes. If they change color, the cat may need some antibiotics.
Long term voice loss could be a result of more serious factors and you should see the vet to rule out an obstruction, cancer, etc.
I am leaning toward the short term causes since you are seeing it in all three cats.
Let me know if you still have questions.
