C#

string sConnection = "Provider = Microsoft.Jet.OLEDB.4.0;" + "Data Source = Schedule.mdb";

            try
            {
                OleDbConnection dbConnect = new OleDbConnection(sConnection);
                dbConnect.Open();

                string sqlString = "SELECT * FROM LogIn WHERE ID = '" + txtBoxLogIn.Text + "' AND Password = '" + txtBoxPassword.Text + "';";

                OleDbCommand dbCmd = new OleDbCommand();
                dbCmd.CommandText = sqlString;
                dbCmd.Connection = dbConnect;

                OleDbDataReader dbReader;
                dbReader = dbCmd.ExecuteReader();

                while (dbReader.Read())
                {
                    if (dbReader[0].ToString() == txtBoxLogIn.Text && dbReader[1].ToString() == txtBoxPassword.Text )
                    {
                        frmMainForm frmMainForm = new frmMainForm();
                        frmMainForm.ShowDialog();
                        this.Close();
                    }
                    else
                    {
                        MessageBox.Show("Either the Username or Password does not exist", "Log-In Error", MessageBoxButtons.OKCancel, MessageBoxIcon.Error);
                    }
                }
                
                dbReader.Close();
                dbConnect.Close();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.ToString());
                return;
            }

class Form3:Form
{
private string m_data;
public string Data
{
get
{
return m_data;
}
set
{
m_data = value;
}
}
}

Form3 form = new Form3();
if(form.ShowDialog() == DialogResult.OK)
{
string dataPassedFromForm3 = form.Data;
}

class Form3: Form
{
  string dataReceived;
  public Form3(string _dataToReceive)
  {
    dataReceived = _dataToReceive;
  }
}