A mana cost of means that you pay twice X. If you want X to be 3, you pay 6 mana to cast Chalice of the Void.

The number of counters on Chalice of the Void matters only at the time the spell is cast. Changing the number of charge counters on Chalice of the Void after a spell has been cast won’t change whether the ability counters the spell. If the Chalice had the correct number of counters when the spell was cast, its ability will trigger. If the Chalice had too many or too few counters when the spell was cast, the Chalice’s ability won’t trigger.

If there are zero charge counters on Chalice of the Void, it counters each spell with a converted mana cost of 0.

Chalice of the Void has to be on the battlefield at the end of casting a spell for the ability to trigger. If you sacrifice Chalice of the Void as a cost to cast a spell, its ability can’t trigger.