calcuation of rate based on values from three dropdownlist values

<asp:DropDownList ID="DropDownList2" runat="server" AutoPostBack="True" AppendDataBoundItems="true" DataSourceID="SqlDataSource1" DataTextField="tripscheme" DataValueField="stdcharge" onselectedindexchanged="DropDownList2_SelectedIndexChanged" ><asp:ListItem Selected="True" Text="- Select Trip -"></asp:ListItem> </asp:DropDownList> <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString1 %>" SelectCommand="SELECT [tripscheme],[stdcharge] FROM [tripscheme]"></asp:SqlDataSource>
<asp:DropDownList ID="DropDownList3" runat="server" AutoPostBack="true" onselectedindexchanged="DropDownList3_SelectedIndexChanged"> <asp:ListItem> ...