I've tried this :

  <div id="xDiv" align="left">
       <div id="divX" runat="server" style="margin-left:40px; width: 650px;" 
           align="center">
  <asp:Menu ID="Menu3" runat="server" Orientation="Horizontal" Width="100%" 
                       StaticSelectedStyle-CssClass="StaticSelectedStyle" Height="52px" 
                       StaticSubMenuIndent="18px" oninit="Menu3_Init">
     <StaticSelectedStyle CssClass="StaticSelectedStyle"></StaticSelectedStyle>
     <Items>
        <asp:MenuItem Text="Test" />
     </Items>
  </asp:Menu>
  </div>
    <div runat="server">
        <asp:DropDownList ID="DropDownList3" runat="server" 
            DataSourceID="SqlDataSource1" DataTextField="RepGroup_Name" 
            DataValueField="RepGroup_ID" Width="162px">
        </asp:DropDownList>
    </div>
  </div>

but it puts DropDownList not to the end of line , it puts it on a new line.