This is supposed to work automagically .. ie extend wireless by ethernet.. but my experience so far has been negative.. it never works.
If you have all the latest gear.. latest AE, latest version Airport Utility on the latest OS. It might work.. then again it might do your latest head in.
Do it manually.. then Apple take over control of your life will stop.
If one airport is working as router.. set that one up first.
That is the easy one.
All the rest do manually.. Plug the Airport into the computer by ethernet with no network link at all to the rest of your gear.
Factory reset. Most important.. start fresh.
Open the airport utility and go to top menu area..

Type in the IP address .. always 10.0.1.1 after a reset. No password is needed. It is public if you ever need it.

This will bring up the real setup pages.. here you are in charge.. you are the MASTER.. you control the network.

Go to each tab..
Each unit requires a different base station name.. all short, no spaces and pure alphanumeric. eg AE1 AE2 AE3 etc.
Simpler the better.
Internet tab should be dhcp which is right by default.
Wireless tab.. create a wireless network.. same name as the first AE1 wireless.. better if it is also short, no spaces and pure alphanumerics.
Same password.. same rules but longer.. 8-20 characters mixed case and numbers.
Same security setting, WPA2 personal is the only one.. !!
Network tab .. bridged.

Click update..
Do the next one..
Keep track of them .. As you finish it plug it in by ethernet and make sure it works..
If it fails at 3 tell me.. something weird is happening.