More info, please. How old are they? Have you wormed them? When, with what? What do they eat?
How is their poop looking? Are you sure there are no other symptoms leading up to the deaths?
Do they free range? They may have eaten something toxic? Three in two months, with almost 40 chickens is more than you would want, but not a lot.
Depending on age.... especially young hens that have only been laying a short time: hens that die in the nest may well have been eggbound, even ones that die in, near, around the nests.
Use the search here on BYC for more info. -- search Eggbound, consider their age -- if laying they are old enough to need worming -- especially if they free range.
Sorry I can't be more help.
Good luck, hope you don't lose anymore, hope it is an anomally.