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.

4. Make a Graphic Style
With the yellow piece selected, in Graphic Styles pallate, click on the New Graphic Style icon to make a Graphic Styles of that object. If your Graphic Styles pallate is not showing, go to Window > Graphic Styles or press Shift+F5.

5. Apply Graphic Styles
Now select the orange and green piece, click on the Graphic Style you’ve created in previous step to apply the styles onto these objects.

Final
One last step you need to do is to reposit the pie. Notice after you applied the Graphic Styles, the pie is off a bit, use the Selection Tool and put them back together. Now you have a nice colorful 3D pie graph.

Bonus Tips
You can use the same technique to create the bar graph like the image below.

About the author:

Nick started http://www.ndesign-studio.com in 2005 where he showcases his work, now N.Design Studio is also a Illustrator community where you can find Illustrator tutorials and free design resources.