Since data disk order doesn't matter with single parity these will assume the missing disk is disk3, you need to be on Unraid 6.2.4 or newer.
-Tools -> New Config -> Apply
-Assign parity plus the other 2 existing data disks, assign the new disk as disk3, double check all assignments.
-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 3 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), disk3 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