.aspx
<asp:Table ID="Table1" runat="server" BorderColor="DarkGreen" BorderWidth="1">
<asp:TableHeaderRow ForeColor="SeaGreen">
<asp:TableHeaderCell>RollNumber</asp:TableHeaderCell>
<asp:TableHeaderCell>Name</asp:TableHeaderCell>
<asp:TableHeaderCell>Mark</asp:TableHeaderCell>
</asp:TableHeaderRow>
<asp:TableRow>
<asp:TableCell>1</asp:TableCell>
<asp:TableCell>ss</asp:TableCell>
<asp:TableCell>33</asp:TableCell>
</asp:TableRow>
</asp:Table>
Code
protected void btnAdd_Click(object sender, EventArgs e)
{
TableRow TRow = new TableRow();
TableCell rollnocell = new TableCell();
rollnocell.Text = txtroll.Text;
TRow.Cells.Add(rollnocell);
TableCell namecell = new TableCell();
namecell.Text = txtname.Text;
TRow.Cells.Add(namecell);
TableCell markcell = new TableCell();
markcell.Text = txtmark.Text;
TRow.Cells.Add(markcell);
Table1.Rows.Add(TRow);
}