It's possible to convert drives to RAID in-place... but strongly discouraged.
Since OP will have a blank drive, they could play musical chairs by setting up a new RAID on the new empty drive, copy data from one drive, wipe that drive, grow the array, copy data from the third drive, wipe, grow... But that's going to take a long time, and you'll have to keep notes about where you are in the process, lest you forget which drive is which over the multiple days this will take.