Cannot set the selectedindexchange of two dropdownlist in gridview

Question

I have three dropdownlist in gridview's template field. If the first dropdownlist has selected value in it then i am populating the other two dropdownlist with an array of listitem. Both the dropdownlist will have same items in them.

Suppose I have dropdown named ddlone, ddltwo, ddlthree. If ddlone has a selectedvalue then according to the selectedvalue of ddlone I am adding items in ddltwo and dllthree. They both have same list of items. The problem is when I set the selectedindex or selectedvalue of either ddltwo or ddlthree then both get the same index or values I don't know how.

Here is my gridview:

<asp:GridView ID="GridView1" runat="server" 
AutoGenerateColumns="false" OnRowDataBound="gridview_RowDataBound">
<HeaderStyle HorizontalAlign="Left" />
<Columns>
    <asp:BoundField HeaderText="SrNo" DataField="SrNo" />
    <asp:TemplateField HeaderText="ONE">
        <ItemTemplate>
            <asp:DropDownList ID="ddlone" runat="server" AutoPostBack="True"
            OnSelectedIndexChanged="ddlone_SelectedIndexChanged">
            </asp:DropDownList>
        </ItemTemplate>
    </asp:TemplateField>
    <asp:TemplateField HeaderText="Two">
        <ItemTemplate>
            <asp:DropDownList ID="ddltwo" runat="server">
            </asp:DropDownList>
        </ItemTemplate>
    </asp:TemplateField>
    <asp:TemplateField HeaderText="Three">
        <ItemTemplate>
            <asp:DropDownList ID="ddlthree" runat="server">
            </asp:DropDownList>
        </ItemTemplate>
    </asp:TemplateField>
    <asp:TemplateField HeaderText="Add">
        <ItemTemplate>
            <asp:LinkButton ID="lnkAddon" runat="server" Text="+" 
              Font-Underline="false">
            </asp:LinkButton>
        </ItemTemplate>
    </asp:TemplateField>
</Columns>
</asp:GridView>

All this is being done in gridview_rowdatabound event here is my code:

ddlone.SelectedValue = value_one;
items = GetItems(value_one);
ddltwo.Items.Clear(); ddlthree.Items.Clear();
foreach (ListItem it in items)
{
    ddlopen.Items.Add(it); ddlclose.Items.Add(it);
}
if (!string.IsNullOrEmpty(value_two)) ddltwo.SelectedValue = value_two;
if (!string.IsNullOrEmpty(value_three)) ddlthree.SelectedValue = value_three;

When the second last statement is executed the value of ddltwo and ddlthree becomes same. When the last statement is executed the selecetedvalue of ddltwo and ddlthree becomes same.

I tried making a copy of items but still the problem persists.