The power cut has corrupted a setting.
Plug the TC into a computer by ethernet. Do a factory reset and start over.
Complete the setup.. then do the AE, again directly by ethernet.
I presume the TC is bridged and plugged into the HH3.. then the express is being used to extend wireless. If not tell us more info.
When you finish the setup.. in the airport utility use the export configuration to save a configuration copy.. if you get stuck again you can then quickly restore the working config.
I am trying to install the TC as an extension of an existing network, mainly for use as a network file sharing/backup device. I have conected the TC thru ethernet directly to my computer and done a hard reset (using the ethernet dock, not the modem dock)... everything seems in order until at the end... i receive the message "unexpected error occurred. try again"