When you get ready to add your third XBee, think carefully about where you're going to put the coordinator. For example if you choose to hook it to a laptop, the network dies (yes, it quits working entirely) when you close the lid on the laptop.
Oh yeah! I just noticed that the mesh network documentation was written by you Jack. Good work! You are a "mesh network" saver!! XD
I just shut down my coordinator, and sure enough, the other devices kept working. Then I shut down one of the other devices (a router) and turned it back on, it couldn't connect. I let it try while I crawled back up into the attic and turned on the coordinator and the disconnected router hooked up in a few seconds. There was an end device sleeping and waking up the entire time that kept working just fine through the whole episode. WTF??
...the network doesn't come back up until the coordinator does...