Charitable Levy's triggered ability resolves before the spell that causes it to trigger. The ability will resolve even if that spell is countered or otherwise leaves the stack.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.

In the rare case where the player who controlled Charitable Levy as its last ability triggered doesn't control it as it resolves, that player won't be able to sacrifice Charitable Levy even if it has three or more collection counters on it.