Your control tags are not closed properly. Please use below corrected code
<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server"></asp:ToolkitScriptManager>
<asp:TextBox ID="TextBox3" runat="server" ReadOnly="true" ></asp:TextBox>
<asp:ImageButton ID="ImageButton1" runat="server" Height="25px" ImageUrl="~/images/calendal.jpg" Width="25px" CausesValidation="true" />
<asp:RangeValidator ID="RangeValidator1" runat="server" ErrorMessage="Invalid date" ControlToValidate="TextBox3" MaximumValue="01/01/2014" MinimumValue="01/01/1900" Type="Date" />
<asp:CalendarExtender ID="CalendarExtender1" runat="server" Format="dd/MM/yyyy"
PopupButtonID="ImageButton1" PopupPosition="BottomRight"
TargetControlID="TextBox3" />