If disk3 is really dead there's not much else you can do, and nothing to lose really, except time, do this:
-Tools -> New Config -> Retain current configuration: All -> Apply
-Assign any missing disk(s), including new disk3
-Important - After checking the assignments leave the browser on that page, the "Main" page.
-Open an SSH session/use the console and type:
mdcmd set invalidslot 3 29
-Back on the GUI and without refreshing the page, just start the array, do not check the "parity is already valid" box, disk3 will start rebuilding, if parity is valid disk should mount immediately but if it's unmountable don't format, wait for the rebuild to finish and then run a filesystem check