Parkin, 30, is on a one-month loan while Gibbons has agreed an 18-month contract.
Mirfin, who left the Iron for Watford in the summer, has returned on loan until the end of the season.
A deal had hinged on whether his parent club Watford would allow him to leave, with the future of their centre-back Adrian Mariappa in doubt.
But he played for the Hornets against Millwall, and looked set to stay put.
The club also said there were no further announcements due, meaning Michael O'Connor was due to stay despite an approach from the Scottish Premier League side Hibernian.