howdy stitch.
Sorry to hear about your hardware failure. This forum post talks about replacing drives What is the process for replacing a bad drive?. TLDR
I believe you should be able to replace the drive in cli
btrfs replace [drive id] [new drive] /mnt2/pool-name
The data that is corrupted you should be able to run scrub and either replace the bad sectors from a snapshot or remove the bad files and replace from a cold storage.
Wiki says that when you place a third drive in raid 1 (mirror) you don’t increase the total striped volume of the pool you just add yet another spare of the data that is maintained across the array essentially giving you 2 failure before you lose everything.