{Collective Brutality}
If it gets countered, do I still need to discard the cards for the additional modes?
When you cast it. You declare all modes you will be using along with the cost. If you declared to use all 3, You have to pay the mana and discard the cards for the escalate.
At this point if they get a chance to respond to the spell and can counter it. As the discard was part of the cost. Your out of luck, those cards are gone.