Adding another row with all the textboxes on add button click

Protected void btnSave_Click(object sender, EventArgs e)
{
           string val1=textbox1.Text;
           string val2=textbox2.Text;
           /// pass those string values as a parameter while insert them into database.
}

Protected void gv_OnRowCommand(object sender, GridViewRowCommandEventArgs e)
{
          if(e.CommandName=="Add")
         {
                     //find your controls here and insert it into database.
         }
}

< asp:gridview ID="Gridview1" runat="server" ShowFooter="true" AutoGenerateColumns="false">
        < Columns>
        < asp:BoundField DataField="RowNumber" HeaderText="Row Number" />
        < asp:TemplateField HeaderText="Header 1">
            < ItemTemplate>
                < asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
            < /ItemTemplate>
        < /asp:TemplateField>
        < asp:TemplateField HeaderText="Header 2">
            < ItemTemplate>
                < asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
            < /ItemTemplate>
        < /asp:TemplateField>
        < asp:TemplateField HeaderText="Header 3">
            < ItemTemplate>
                 < asp:TextBox ID="TextBox3" runat="server"></asp:TextBox>
            < /ItemTemplate>
            < FooterStyle HorizontalAlign="Right" />
            < FooterTemplate>
             < asp:Button ID="ButtonAdd" runat="server" Text="Add New Row" />
            < /FooterTemplate>
        < /asp:TemplateField>
        < /Columns>
< /asp:gridview>

//aspx.cs
private void SetInitialRow()
    {
        DataTable dt = new DataTable();
        DataRow dr = null;
        dt.Columns.Add(new DataColumn("RowNumber", typeof(string)));
        dt.Columns.Add(new DataColumn("Column1", typeof(string)));
        dt.Columns.Add(new DataColumn("Column2", typeof(string)));
        dt.Columns.Add(new DataColumn("Column3", typeof(string)));
        dr = dt.NewRow();
        dr["RowNumber"] = 1;
        dr["Column1"] = string.Empty;
        dr["Column2"] = string.Empty;
        dr["Column3"] = string.Empty;
        dt.Rows.Add(dr);
        //dr = dt.NewRow();
         //Store the DataTable in ViewState
        ViewState["CurrentTable"] = dt;
         Gridview1.DataSource = dt;
        Gridview1.DataBind();
    }