Answer: 

Divide the berries into three groups of three. Weight two of these groups and set the third group aside. If the one group is heavier than the other you know which group contains the heaviest berry. If both groups balance then the heaviest berry is in the third group.

Take the group with the heaviest berry and put two of the berries on the scale and set the third berry aside. If scales do not balance you know which berry is the heaviest. If the scales balance, then the third berry is the heaviest.