This was them yesterday it is the one that is leaned against the one standing on the right side that died
So sorry. 
I apologize if you've already said. .but what are you feeding them?
Also Check their vents for pasty butt. I lost one duckling to what I think was pasty butt this year, and fought it with two others for several days. Basically that's when their poo is thick and sticky and builds up on their bum preventing them from relieving themselves and they can eventually die if it goes unnoticed.

