The things that spring to mind are things like lamb dysentry, or especially in the case of healthy looking older lambs just dropping dead, pulpy kidney, although if they are from vaccinated stock then vaccinated when old enough, shouldn't be an issue. With those level of losses, I know it is sometimes hard to throw good money after bad, but I would be getting PMs done to identify the cause, for now and future years, and as Sally says, if they are not vaccinated, do so, I BELIEVE, although I would confirm with a vet rather than take my word for it, that although 3 weeks is the recommened age for their first jab they can have it at 2. Hope things start to look up for you soon.