Couple of options, the others may have been predated/ died, or hopefully, they may have migrated already
The nest is in our small boiler house, next to the back door and kitchen. I love watching from the kitchen window the parents, and eventually fledglings, flying in and out. The brood was quite late in the summer, and now today which is very windy, wet and cold, there are 3 of them which have flown in to escape the wind and rain! Here’s an earlier photo taken after they had all fledged. The nest is up on the beams next to the light which of course is never put on when they are here. Also, on strict orders from me, no one is allowed to go in when they’re there all simmer either! Here’s a photo quickly taken when the first 2 moved out of the nest for a while.