Replace you Panel3 tag with following
<asp:panel id="Panel2" runat="server" style="width: 550px; overflow-x: auto; overflow-y: auto;height: 400px;" xmlns:asp="#unknown"></asp:panel>
This will set the height of your control to 400px and width to 550px. You can change it. Now in anycase if width or height exceeds these values a scroll will appear.