My interpretation after carefully reading the original post was that 3 racquets were strung during the evening with just the mains. That means the racquets were removed from the stringer and sat around overnight. The next day he finished doing the crosses on the other 2 racquets. On the 2nd day when he came home to finish the 3rd, that's when he saw it broken.
Major mistake in your string jobs. Hope these are your racquets and not for customers or friends. You should NEVER remove a racquet from a stringer until it is completed. The 2 or 6 point mounts are there for a reason and that is to prevent or reduce the chances of the tennis racquet frame from distorting. By removing the racquets with only the mains strung, I guarantee you the frames are slightly distorted in that it will not measure 27" exactly. It could be off by 1/8" or more. All depends on how stiff the frame is.
This is a hard lesson learned but you should cut out the string job and restring the racquets. I know you will never make this mistake again.