You can't put back original disk while disk status is invalid, you can do this, assuming parity is still valid with old disk3:
-Tools -> New Config -> Retain current configuration: All -> Apply
-Assign any missing disk(s) if needed, including old disk3 and new disk2
-Important - After checking the assignments leave the browser on that page, the "Main" page.
-Open an SSH session/use the console and type (don't copy/paste directly from the forum, as sometimes it can insert extra characters):
mdcmd set invalidslot 2 29
-Back on the GUI and without refreshing the page, just start the array, do not check the "parity is already valid" box (GUI will still show that data on parity disk(s) will be overwritten, this is normal as it doesn't account for the invalid slot command, but they won't be as long as the procedure was correctly done), disk2 will start rebuilding, disk should mount immediately but if it's unmountable don't format, wait for the rebuild to finish and then run a filesystem check
Keep old disk2 intact for now in case it's needed.