private void dataGridView1_CellValueChanged(object sender, DataGridViewCellEventArgs e)
    {
        bool nullCell = false;
        for (int j = 0; j < dataGridView1.Rows.Count; j++)
        {
            for (int i = 0; i < dataGridView1.Rows[j].Cells.Count; i++)
            {
                if (dataGridView1.Rows[j].Cells[i].Value == null)
                {
                    nullCell = true;
                }
            }
        }
        if (!nullCell)
        {
            button3.Enabled = true;
        }
    }