My guess yesterday was that the third egg had been laid, and that that was what had shifted Mme. Owl into serious brooding mode. Unfortunately, I wasn't able to capture an overhead view yesterday to confirm that guess. This evening, however, I was able to capture such a view, and, as you’ve undoubtedly already noticed, there is a third egg. Whether it was laid yesterday or today will likely remain a matter of guesswork, but there it is.
As always, if this sort of thing interests you, you can watch the nesting proceed in real-time from my nest box cam’ site.