The default pie drawing behavior in .NET does not create a pie slice that has 3d perspective when the bounds are rectangular. In order to give the pie slice 3d perspective you need to transform the angles using a little trig. I'm not a math expert so I'm not going to attempt to explan why/how the formulas work...  Kudos to a ton of math sites and Julijan Sribar for his 3d pie chart for helping me figure this out. The following functions allow you to compute the angles for a 3d perspective:

Private Function To3dSweepAngle(ByVal Bounds As Rectangle, _
   ByVal Angle As Single, _
   ByVal SweepAngle As Single) As Single

   If SweepAngle Mod 180 <> 0 Then

      Dim Angle3d As Single = To3dAngle(Bounds, Angle)
      SweepAngle = To3dAngle(Bounds, Angle + SweepAngle) - Angle3d

   End If

   If SweepAngle < 0 Then

      SweepAngle += 360

   End If

   Return SweepAngle

End Function

Private Function To3dAngle(ByVal Bounds As System.Drawing.Rectangle, _
   ByVal Angle As Single) As Single

   Dim Radians As Single = ToRadian(Angle)
   Dim X As Double = Bounds.Width * Math.Cos(Radians)
   Dim Y As Double = Bounds.Height * Math.Sin(Radians)
   Dim Angle3D As Single = Math.Atan2(Y, X) * 180 / Math.PI

   If Angle3D < 0 Then

      Return Angle3D + 360

   Else

      Return Angle3D

   End If

End Function

Private Function ToRadian(ByVal Angle As Single) As Single

   Return (Math.PI * Angle) / 180

End Function

The following code snipet demonstrates how to draw a pie slice with 3d perspective:

Private Sub Form1_Paint(ByVal sender As Object, _
   ByVal e As System.Windows.Forms.PaintEventArgs) Handles Me.Paint

   Dim PieBounds As New Rectangle(20, 20, 400, 100)
   Dim PieAngle As Single = 150
   Dim PieSweepAngle As Single = 100

   '--> Determine the 3d angles
   PieSweepAngle = To3dSweepAngle(PieBounds, PieAngle, PieSweepAngle)
   PieAngle = To3dAngle(PieBounds, PieAngle)

   '--> Draw the pie
   e.Graphics.FillPie(Brushes.SteelBlue, PieBounds, PieAngle, PieSweepAngle)

End Sub