Found one good egg and one smashed and eaten egg in the nest box yesterday.
I'm not sure which of the three hens is doing this. May be more than one of them.
So yesterday, I took two good eggs from my kitchen counter and poked a hole in each end with a bulletin board push pin.
Then blew the contents of each egg into a frying pan and cooked up my lunch.
Then put some jarred horseradish and yellow mustard and a bit of water into a small food processor and pureed it all into a thin consistency.
Used an eyedropper and filled both eggs about halfway full of that stuff.
Cut open a butternut squash and waited twenty minutes for the sticky juice to ooze out onto the cut surface of the squash.
Smeared some of that stickiness onto the pierced holes of the two eggs, waited a few hours til it was good and dry, and gently placed the two eggs in the nest box late yesterday afternoon.
Ten o'clock this morning, found both doctored eggs intact, and one new egg all smashed and eaten.
Considering my options:
Just wait and see if any of them try eating the doctored eggs.
Isolate one of the three in a separate coop and see if either coop's nest box presents me with smashed and eaten eggs over the next few days. (The separate coop is right next to the main coop, so they'll all still be able to see and talk to each other, so the stress of separation will be less than if it were total isolation.)
Try rigging up a slanted floor for the nest box so the eggs can immediately roll away downhill beyond a barrier so the hen can't reach it.
I'm not sure which of the three hens is doing this. May be more than one of them.
So yesterday, I took two good eggs from my kitchen counter and poked a hole in each end with a bulletin board push pin.
Then blew the contents of each egg into a frying pan and cooked up my lunch.
Then put some jarred horseradish and yellow mustard and a bit of water into a small food processor and pureed it all into a thin consistency.
Used an eyedropper and filled both eggs about halfway full of that stuff.
Cut open a butternut squash and waited twenty minutes for the sticky juice to ooze out onto the cut surface of the squash.
Smeared some of that stickiness onto the pierced holes of the two eggs, waited a few hours til it was good and dry, and gently placed the two eggs in the nest box late yesterday afternoon.
Ten o'clock this morning, found both doctored eggs intact, and one new egg all smashed and eaten.

Considering my options:
Just wait and see if any of them try eating the doctored eggs.
Isolate one of the three in a separate coop and see if either coop's nest box presents me with smashed and eaten eggs over the next few days. (The separate coop is right next to the main coop, so they'll all still be able to see and talk to each other, so the stress of separation will be less than if it were total isolation.)
Try rigging up a slanted floor for the nest box so the eggs can immediately roll away downhill beyond a barrier so the hen can't reach it.