Is it advisable for me to perform factory resets on the Airport Expresses before reconfiguring them into a roaming network?
Yes, since Apple's configuration "wizard" in AirPort Utility requires that the AirPorts to be reconfigured are in a factory default state.
Any other tips and tricks to look out for?
Assuming that you have tested to make sure that the Ethernet connections are working correctly, the setup process is extremely simple and straightforward.
Reset each Express back to factory default settings
Connect the Ethernet cable to the WAN "O" port on the AirPort Express
Click the WiFi menu on your Mac and then click on AirPort Express under the listing of new AirPort Base Station
When you click on AirPort Express, the setup "wizard" will take a few seconds to automatically suggest the correct setup
Type in a device name that you want to call the AirPort Express
Click Next, and the wizard will set up everything for you correctly and automatically
Cick Done when you see the message of Setup Complete....and you are done with that Express
Set up the other two Expresses one at a time exactly the same way
