Smith was not the only hat-trick hero, as Daffyd James and Jamie Henthorne matched his feat, and Rob Smith came close after crossing twice.
Club captain Jack Dawson, playing his 100th game for the club, capped off a dream day by grabbing a try, along with Greg Jones, Darren Pearce and David Ford.
Garndiffaith also took a league title, winning both promotion and Division Three East on the same day after a 38-17 win over Gwernyfed.
Garndiffaith, who lifted the trophy at their Lasgarn View home afterwards, ran in six tries with scrum-half Rhodri Jones notching two. Andrew Gunter, Gareth Jones, Matthew Billings and Chris Baldwin picked up the rest.
Llandaff haven't quite guaranteed promotion from Division Four East, but they are now mighty close to doing so after their win at Crumlin. Ezra Huckle, Adrian Walsh and Barry Greenley all touched down for tries in a 24-21 victory.
With Nantyglo slipping up at Brynithel 26-21, Llandaff know a draw against Risca on Tuesday night would be enough to see them up. Abercarn aren't up yet in Division Five East, after they were beaten 20-3 by second-placed Talywain.