Cannot find HtmlGenericControl (with runat="server") using FindControl

Hello, I'm sure I'm doing something wrong here, but I cannot figure it out. Please help.
This is in my aspx page:
<asp:Content ID="MainContent" ContentPlaceHolderID="MainContentPlaceHolder" runat="Server">
    <div id="Div1" runat="server" visible="false">xxx</div>
    <div id="Div2" runat="server" visible="false">yyy</div>
    <div id="Div3" runat="server" visible="false">zzz</div>
</asp:Content>
And this is in my aspx.vb page:
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    If Not Page.IsPostBack Then
        Dim DivId As String = Request.QueryString("id")
        ...