In the software, you edit the existing W-2, with all of it's original data, and then make changes to just the boxes were changed in the two W-2C forms...ending up with the last W-2c you received.
Then, on the page that follows the main software form, you'll see a page with a whole bunch of checkboxes, you check a box 2/3rds of the way down to indicate it was a "Corrected W-2".
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*