Power down the TS, insert the new disk, reboot the TS, logon to its interface page, a messsage will come up saying "an error has occured in raid array1" select this message, place and tick beside disk 3 and click restructure raid array. This will go about implementing the disk into the existing raid (which will take several hours)
Once done the unit should be fine.
I would backup your data just incase you loss another disk though :P The chances of it happening are slight but you never know!