[Milhouse]

I've got 3 pis and would like to be sure they all update at approximately the same time. Is there a way to share 1 update directory and just issue sequential reboots?

No, you can't share the .update folder as it is located within the /storage folder, each of which will specific to your 3 Pis. Plus, and assuming you where sharing a single /storage folder between all your Pis, the first Pi to apply the update will delete the update files so that it doesn't enter an "update loop", preventing the remaining Pis from updating.

I suppose you could set each of the Pis to "manual" update, extract the target SYSTEM+KERNEL+MD5 files from a the new package and copy them into the .update folder for each Pi, and they Pi's will update when they next reboot.

Or, have one of the Pi's set to auto-download ("auto" update mode) so that it downloads the new update, and run a script on your NAS server that watches for new files in the .update folder of this Pi and, when they appear, copies them to the .update folder of the other two Pis (these would both be set to "manual" update).