Two female's care for their owlets together in the nest
Perhaps a scarcity of males prompted this threesome. There are plenty of nest sites on his property, including three owl boxes, and quite a few old dairy barns and silos remain in the vicinity, so lack of nest sites shouldn't be the reason these two females are sharing a nest box.
“Voles are extremely abundant on our property, but this seems always to be the case. Abundant prey may be an enabler, but not likely the cause of polygamy.”
Click on the links for more pictures and read the article
http://www.americanartifacts.com/smma/per/nestcam/owlcam06.htm