おそらく、DropDownList コントロールでも「onSelectIndexChanged」を使用する必要があると思います。何かのようなもの...
<asp:DropDownList id="ddl1" runat="server" OnSelectedIndexChanged="ddl1_OnSelectedIndexChanged"></asp:DropDownList>
<asp:DropDownList id="ddl2" runat="server" OnSelectedIndexChanged="ddl2_OnSelectedIndexChanged"></asp:DropDownList>
<asp:DropDownList id="ddl3" runat="server" OnSelectedIndexChanged="ddl3_OnSelectedIndexChanged"></asp:DropDownList>
Page_Load()
{
if(!IsPostBack)
{
ddl1.DataSource = getdata();
ddl1.DataBind();
}
protected void ddl1_onSelectedIndexChanged(object sender, EventArgs e)
{
ddl2.DataSource = getData(ddl1.SelectedValue);
ddl2.DataBind()
}
protected void ddl1_onSelectedIndexChanged(object sender, EventArgs e)
{
ddl3.DataSource = getData(ddl2.SelectedValue);
ddl3.DataBind()
}
protected void ddl3_onSelectedIndexChanged(object sender, EventArgs e)
{
Response.Redirect("SomePage.aspx?ddl1="+ddl1.SelectedValue+"&ddl2="+ddl2.SelectedValue+"&ddl3="+ddl3.SelectedValue, true);
}