However, they had to come back from a 10-0 half time deficit as the Dragons ran in a try from James Lewis and added five points from the boot of outside half Rhys Jones.
The second half was a different story altogether with the Scarlets scoring 17 points without reply.
Rhys Priestland kicked a penalty for the home side and that was followed by tries from wing Kieran Davies and centre John Davies. Priestland converted both before Dragons hooker Neil Williams was yellow carded.
Their opponents in the final will depend upon who wins the final game of the season - on February 22nd - between the Ospreys and the Dragons.
The Ospreys, who have been in excellent form of late, leapfrogged the much fancied Dragons thanks to a 16-13 victory over the Blues at Abercynon.
The Blues actually led by 13 points going into the final quarter thanks to a try, conversion and two penalties from outside half Christian Bala.
However, a Rhys Lawrence try and 11 points from the boot of Nicky Roberts gave the Ospreys the points. Victory leaves them a point clear of the Dragons with the winner of the final game at Abertillery certain to go through.
At under 16 level the table-topping Scarlets went down 7-3 to the Blues South at Pentyrch with Daniel Evans bagging a penalty for the Scarlets.
In the other game, the Ospreys, who are third defeated the Dragins 25-11 at Chepstow.
The Ospreys led 10-8 at half time with tries from Lee Halfpenny and Tom Williams and second half tries from Liam Osowicz (2) and Rhys Webb made sure of the points.
With three games remaining, the Ospreys need at least two victories to stand any chance of reaching the finals in April.
Meanwhile tonight's under 20s game between the Scarlets and the Dragons has been switched to Rodney Parade from Eugene Cross Park. The Dragons will be looking to avenge the defeat they suffered earlier in the season at Stradey Park, but will certainly be up against it.
The Ospreys entertain the Blues at Aberavon where Reebok's ambassador for the competition Rob Howley will be amongst the crowd.