Defend the borders
What happens to the resources stacked on this card? Do you have to spend them to activate the ability?

When a unit questing on a quest is removed from the quest (forced movement, discard, sacrifice, battle damage), all resources tokens on the quest are removed as well.

Therefore if a quest forces you to sacrifice the unit to use the quest, then all resources are removed.

This specific quest card does not involve you removing quest tokens from it. The effect is active as long as you have 3 tokens on it.