string selectSQL = "Select a.CIS FROM ptCIS_Person a, ptCIS_Tel b, ptCIS_ProductMember c where a.CIS = b.CIS and b.CIS = c.CIS and c.MPAcc = '"+70046498+"'";
and then check
if (ds.Table.Count > 0)
{
TextBox1.Text = ds.Tables[0].Rows[0][1].ToString();
}