reload/updet only part of the page on OnSelectedIndexChanged

I have a panelA with a drop down list (ddl1) and a panelB with 2 dropdown lists (ddl2 and ddl3).
I am disabling PanelB in the page_Load section. When a selection has been made in the ddl1 in panelA, I am enabling panelB in OnSelectedIndexChanged event to enable ddl2 and ddl3.
This process is working, but as soon as I make a selection on ddl1, the whole page reloads to enable ddl2 and ddl3, which feels odd. How can I make it just reload panel3 not the whole page? I guess it can be done using AJAX update panel but as soon as I put <%@ Register Assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" tagprefix="ajaxtoolkit" %> and <asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>, I start getting wierd error: Microsoft JScript runtime error: Sys.ArgumentTypeException: Object of type 'Object' cannot be converted to type 'Array'.
Parameter name: array
Pl ...