Libero Back Row Attack?
 

Libero...in front of the attack line....below the height of the net.....3rd hit sends it over the net with a one hand overhead hit......legal or illegal?

No attack, by ANY player, at ANY time, should be ruled illegal if not contacted entirely above the height of the net.

However, remember that a libero may not complete an attack from ANYWHERE if the ball is entirely above the net, while a back row player may complete an attack from behind the attack line with the ball entirely above the net.