I just replayed CW2 and in the ending it said Aliana and Dax's ship goes missing... This explains Lia in 3 but what happened to Dax? Did someone forget to wake him up or something?
Well he probably died of old age? Since Lia and Skars are some kind of immortal space monsters they could survive, but Dax is a regular human so too bad for him.
http://knucklecracker.com/forums/index.php?topic=15480.0 is one explanation...