1. Create the graphic

Select the Pie Graph Tool , click on the Artboard/screen, then enter 200x200px.

Data Table

Your graph data table will pop up like the image below. Enter 15, 55, 30 and then click on the check mark to apply data. You may close this data table now.

2. Ungroup the graph

You need to ungroup or break the graph in order to apply color and effects to each piece. With the graph selected, go to Object > Ungroup or press Ctrl+Shift+G. There will be a warning box(After a graph is ungrouped you will no longer be able to access its graph style, its data or change its graph designs.) just click Yes. Now Ungroup again, first Ungroup is to break the graph, second Ungroup is to break the object into pieces. Fill with any color as you want and remove stroke color.

3. Apply Extrude & Bevel

Select any piece(I've selected yellow here), go to Effect > 3D > Extrude & Bevel.

3D Extrude & Bevel options

If the lower portion window is not showing, click More Options. Now you may drag the cube on the left to specify the Rotation or enter the X-Y-Z Axis values manually. Enter 20pt for Extrude Depth. Add an extra Light by clicking on New Light icon and drag it as shown below.