How to add multiple rows to a Gridview dynamically

<asp:GridView ID="GridView1" runat="server" Font-Names="Arial" Font-Size="XX-Small"
Style="z-index: 118; left: 174px; position: absolute; top: 421px" Width="543px" Height="66px" EmptyDataText="--- Empty Grid ---" AllowPaging="True" Caption="Customer Status Report" CaptionAlign="Top" DataKeyNames="Item #" >
</asp:GridView>
<asp:textbox ID="Textbox1" runat="server"></asp:textbox>
<asp:textbox ID="Textbox2" runat="server"></asp:textbox>
<asp:Button ID="Button2" runat="server" Text="Begin" OnClick="Button2_Click" />
<asp:Button ID="Button3" runat="server" Text="Add new" OnClick="Button3_Click"
style="z-index: 1;font-family:Sans-Serif; height:22px; left:122px;  margin-top:100px; font-weight:900; "  />

protected void Button2_Click(object sender, EventArgs e)
{
    DataTable dt = new DataTable();
    dt.Columns.Add(new DataColumn("Item #", typeof(int)));
    dt.Columns.Add(new DataColumn("Customer Name", typeof(string)));
    dt.Columns.Add(new DataColumn("Contract Number", typeof(string)));
    DataRow dr = dt.NewRow();
    dr["Item #"] = 1;// i;
    dr["Customer Name"] = Textbox1.Text;
    dr["Contract Number"] = Textbox2.Text;
    dt.Rows.Add(dr);
    this.GridView1.Visible = true;
    GridView1.DataSource = dt;
    GridView1.DataBind();
}
protected void Button3_Click(object sender, EventArgs e) //to add new rows to GV
{
}