Divide 27 bags into 3 groups, each group

27÷3=9 bags, divided into two groups and weighed once. If they are the same weight, the extra bags are in another group, or whichever side is heavier, find which group and divide them into three groups. 9÷3=3 bags. The method is as above. Finally, divide the three bags into three groups, and then weigh them, and you can find them.