The Dragons were the only Welsh region to have conjured up a victory in the tournament in the opening two rounds and two tries from former Wales wing Dafydd Howells edged them into a 17-14 lead in the first half at the Arms Park.
But the Blues had tries from Dan Fish, Harri Millard and Ben Murphy to thank for going into the break 21-17 ahead. They added three more in the second half from Lewis Jones (2) and Rory Thornton.
That kept edging them ahead, but the Dragons refused to go away and scrum halves Tavis Knoyle and Rhodri Williams crossed for tries to make it 10 in the match. Prop Ryan Bevington got the Dragons’ opening try.
There were also 10 tries at Parc Y Scarlets as the home side ran out convincing winners. Harry Doel grabbed two of the Scarlets’s eight touchdowns in the second half after they had built up a 24 point interval lead.
Prop Steff Thomas, who picked up a yellow card just before the break in an altercation with Ospreys front rower Jordan Walters that saw him receive a red, got the first and Shaun Evans and Dafydd Land got the others before the break.
Javan Sebastan, Aron Hemmings and Jac Morgan joined Doel in crossing the Ospreys’ line in the second half and Ioan Hughes added six conversions and a penalty. Callum Carson and Cam Lewis scored for the Ospreys.