Help! 2 hens dead in 2 days; 3rd sick

We raised 9 white Leghorns from a hatchery, and they're 1 year and 4 months old. The second one we found dead in the coop just now, and a third is sick. They don't free-range, but they have a very large coop, roomy hen house and are under large shade trees in our backyard. Hubby just noticed that their poop looks very runny and isn't formed, maybe even closer to diarrhea. Help! We're afraid we're going to lose them all in a short period of time!