The Pie chart widget allows you to specify the source of the object you are displaying. Where are the 3 objects that you want to show located?
If they are the only 3 objects in your database, you can choose Datasource → Database. You can also choose to select or create a microflow to retrieve the 3 objects and select them via Datasource → Microflow. You can also retrieve your objects through a REST endpoint from another server (Datasource → REST endpoint).
Then on the Data points tab, you can select which value should be displayed under Value attribute, and you can select the name to be displayed under the Name attribute.
Hope this helps!
You are specifying where the data will be retrieved, then all the objects from your entity will be shown in the Pie chart.
If you are looking for only three make sure that you don't have more, or limit them using the Microflow as a data source.