Heya,
I think you have something mixed.
Lets assume the Dauntless receives 3 hits from an enemy Murder Class Cruiser. Just, plain regular weapon battery hits.
The first hit goes against the single shield of the Dauntless, place a blastmarker in contact with the Dauntless to show.
The second and third hit inflict damage on the Dauntless. Reducing it from 6 to 4 hitpoints.
Now, roll for each inflicted point of damage a dice. On a 6+ a critical hit is inflicted. Roll on the Critical damage chart.
(If the Dauntless was on Brace for Impact it could save against the hits causing damage, not against the hit versus the shield).