i have 3 dropdowns and populating with 3 tier architeture

Try this,
Here,Three AJAX Cascading Dropdowns along with the three corresponding ASP.Net DropDownLists will be populated from SQL Server database with values of Country State City using Web Service and Web Methods.
http://www.aspsnippets.com/Articles/AJAX-Cascading-DropDownList-sample-with-database-using-ASPNet.aspx[^]

Hi,
Create one webApplication, and two classLibrary.
In Web Application:
Code In Aspx Page:

<form id="form1" runat="server">
    <div>
    
        <asp:dropdownlist id="Country" runat="server" xmlns:asp="#unknown">
            onselectedindexchanged="CountryValueSelectedIndexChanged" 
            AutoPostBack="True">
        </asp:dropdownlist>
        <br />
        <asp:dropdownlist id="State" runat="server" xmlns:asp="#unknown">
        </asp:dropdownlist>
    
    </div>
    </form>

Code with in Aspx.cs Page:

BusinessLogic businessLogic = new BusinessLogic();
       protected void Page_Load(object sender, EventArgs e)
       {
           if (!IsPostBack)
           {
               CountryBind();
           }
       }
       public void CountryBind()
       {
           DataSet countryDataSet = businessLogic.CountryValues();
           Country.DataSource = countryDataSet;
           Country.DataTextField = "CountryName";
           Country.DataValueField = "ID";
           Country.DataBind();
           Country.Items.Insert(0,new ListItem("-- Select --", "0"));
       }

       protected void CountryValueSelectedIndexChanged(object sender, EventArgs e)
       {
           int countryid = Convert.ToInt32(Country.SelectedValue);
           DataSet countryDataSet = businessLogic.selectState(Country.SelectedIndex);
           State.DataSource = countryDataSet;
           State.DataTextField = "StateName";
           State.DataValueField = "CountryCode";
           State.DataBind();
       }

Create one class Library named as BusinessLayer. With in that rename Program.cs to BusinessLogic.cs.
Code with in BusinessLogic.cs is:

public class BusinessLogic
   {
       DataLogic dbProgram = new DataLogic();
       public DataSet selectState(int countryValue)
       {

           return  dbProgram.returnState(countryValue);


       }
       public DataSet CountryValues()
       {
           return dbProgram.Countries();
       }
   }

Create one class Library named as DataLayer. With in that rename Program.cs to DataLogic.cs.
Code with in DataLogic.cs is:

public class DataLogic
   {
       SqlConnection con = new SqlConnection("Data Source=HYD-NPULIVARTHY;Initial Catalog=test2;Integrated Security=True");
       public DataSet returnState(int countryId)
       {
          // SqlConnection con = new SqlConnection("Data Source=HYD-NPULIVARTHY;Initial Catalog=test2;Integrated Security=True");
           con.Open();
           SqlCommand cmd = new SqlCommand("select  * from State where CountryCode='" + countryId + "'", con);

           cmd.CommandType = CommandType.Text;

           SqlDataAdapter da = new SqlDataAdapter(cmd);
           DataSet ds = new DataSet();
           cmd.ExecuteNonQuery();
           da.Fill(ds);
           con.Close();

           return ds;


       }
       public DataSet Countries()
       {
           con.Open();
           SqlCommand cmd = new SqlCommand("select * from CountryState", con);

           cmd.CommandType = CommandType.Text;

           SqlDataAdapter da = new SqlDataAdapter(cmd);
           DataSet ds = new DataSet();
           cmd.ExecuteNonQuery();
           da.Fill(ds);
           con.Close();

           return ds;
       }
   }

After creating creating 3 Layers, goto Web Application then goto References.
RightClick on the References-> select Project and select BusinessLayer Reference.
Goto BusinessLayer then goto References.
RightClick on the References-> select Project and select DataLayer Reference.

This is the way to implement.