Replace your function from below it will work fine
public void Bind()
{
if(ViewState["dt"]==null)
{
DataTable dt = new DataTable();
dt.Columns.Add("Sr");
dt.Columns.Add("Name");
dt.Columns.Add("Add");
ViewState["dt"]=dt;
}
DataTable dt=ViewState["dt"] as DataTable;
DataRow dr = dt.NewRow();
dr["Sr"] = TextBox1.Text;
dr["Name"] = TextBox2.Text;
dr["Add"] = TextBox3.Text;
dt.Rows.Add(dr);
GridView1.DataSource = dt;
GridView1.DataBind();
}