take three of them and weigh them against three unmarked coins.

Now, take three of them and weigh them against three unmarked coins.If they balance, the remaining unmarked coin must be the fake.If they don't, draw a plus on the three unmarked coins if they're heavier or a minus if they're lighter.Now, take two of the newly marked coins and weigh them against each other.If they balance, the third coin is fake.Otherwise, look at their marks.