Validation with 3 dropdowns

Hi,
I want to add validation for 3 dropdowns

<asp:DropDownList ID="ddlFirst" runat="server"> <asp:ListItem Value="1" Text="1"></asp:ListItem> <asp:ListItem Value="2" Text="2"></asp:ListItem> <asp:ListItem Value="3" Text="3"></asp:ListItem> </asp:DropDownList><asp:DropDownList ID="ddlSeccond" runat="server">                                <asp:ListItem Value="1" Text="1"></asp:ListItem>                 &nbs ...