Suppose you have 12 coins and ONE of them is a counterfeit. You know that the counterfeit coin WEIGHS LESS than the real coins.
Your only tool for measuring the weight is a balance scale, and you are only allowed to use it THREE TIMES.

Can you determine the counterfeit coin with certainty?
How would you do it?

What if you DON'T KNOW whether the counterfeit coin weighs MORE or LESS.
Is it still possible to determine the coin with certainty in only three uses of the scale? How?