Need to Create a calculated column in the grid

Hi,
I have three dropdowns present in the Grid. C,O,S are the dropdowns
I have a column called "Total".
I need to calculate the selected values of all the three dropdowns and bind it to total.
Here is my code:
<asp:TemplateField HeaderText="C"> <ItemTemplate> <asp:DropDownList ID="ddlC" runat="server"> <asp:ListItem>0</asp:ListItem> <asp:ListItem>1</asp:ListItem> <asp:ListItem>2</asp:ListItem> <asp:ListItem>3</asp:ListItem> <asp:ListItem>4</asp:ListItem> <asp:ListItem>5</asp:ListItem> </asp:DropDownList> </ItemTemplate> </asp:Tem ...