First weighing: 9 on the left and 9 on the right. If it is the same weight, the fake ones are in the remaining 9 boxes; The weight is different, and the fake ones are among the nine light ones.

Second weighing: Of the 9 counterfeit coins, 3 are on the left and 3 are on the right. The method is the same as the first time, and you can know that there are three holidays.

Third weighing: There are only three left at this time, 65438 on the left and one on the right. The method is to weigh first, and the lighter one is counterfeit money.