Follow the following steps.
1. Select 'Custom' in the Sort option.
2. Select 'Sort Numerically' in the given list of Custom Sort.
3. Select 'Descending' in the drop down list.
4. Go to the 'Data' tab and in the dimension drop down, select Limitation as 'Fixed Dumber'.
5. Select Top or Bottom according to your requirement and specify the number of Top or Bottom values you want to see in the bar chart. (In your case select Top and give the number as 3)