As you can see, the third egg is out of the nest. But all is not lost, as one of the advantages of having nest boxes next to opening windows, and the nest boxes themselves having opening side doors, is that you can wait until the both birds vacate. They did so shortly after this video and we were able to whizz upstairs and replace the third egg in the nest concave. Hopefully it'll stay there this time - if not we'll have to do it all again!
The third egg now replaced in the nest: