I have an airport extreme as my main WIFI router and I have an Airport Express downstairs which is set to extend the network and it works flawlessly. I recently also got a second Airport Express to extend the Airport Extreme network at my top floor and it always tells me it cannot join the network with the exact same settings as the other Airport Express. At the location where the upstairs Airport Express is set up, the network signal strength is good.
My question is: Is it even possible to extend an Airport Extreme network twice with two different Airport Express routers, or is this not supported?
The error I get is the following. 
Best Answer
I recently was able to do this successfully, adding a 2nd Airport Express with a wifi-only connection to an existing network which had RS232 lines to a modern Airport Extreme and an Airport Express. I'm using the latest Mac OS. What I found most effective after plugging the new unit it was to restart the Mac (Mac Mini circa 2012) several times so that it would, eventually, recognize the new Express hardware. I did set the TCP/IP Configure IPv6 to link-local only but I do not know if that made any difference. Once everything was working I changed back to Configure iPv6 automatically.