how to fill database and textbox value same row datagrid

Question

I want to fill first 3 columns based on combobox select other 2 columns from textbox values

 

I done code in combobox seletectedindexchanged event. but it fills data to next row

private void comboBox3_SelectedIndexChanged(object sender, EventArgs e)
{
OleDbConnection con = new OleDbConnection(@"Provider=SQLOLEDB;Data Source=DESKTOP\SQLEXPRESS;User ID=sa; Password = REGENT; Initial Catalog=Meena; Integrated secutity=SSPI");
DataSet ds = new DataSet();
DataTable dt = new DataTable();
con.Open();
OleDbCommand cmd = new OleDbCommand(("Select t1.itemcode, t2.Description, t1.UOM from Item t1 INNER JOIN ItemGroup t2 ON t1.Itemcode = '" + comboBox3.Text + "'"), con);
OleDbDataAdapter da = new OleDbDataAdapter();
da.SelectCommand = cmd;
DataRow dr = dt.NewRow();
da.Fill(dt);
dt.Columns.Add("Quantity");
dt.Columns.Add("Rate");
dt.Columns.Add("Amount");
dr[3] = textBox3.Text;
dr[4] = textBox5.Text;
dt.Rows.Add(dr);
if (dt.Rows.Count > 0)
{
dataGridView1.DataSource = dt;
}
}