How can we retrive 3 combobox values and show it together in a textbox

protected void btnClick(object sender,EventArgs e)

{
     string Id=chkselect1.SelectedItem.ToString()+"/"+chkselect2.SelectedItem.ToString()+"/"+....
}

private string GetLastRegistrationNumberForCourse(string course)
{
 string registrationNumber = string.Empty;
 // create connection, command, datareader etc and get the last registration number by 
 // passing the course name or id
 return string.Format(registrationNumber, ("0000"));
}

private void cmbCourse_SelectedIndexChanged(object sender, EventArgs e)
{
  string registrationNumber = GetLastRegistrationNumberForCourse(cmbCourse.Text);
  txtEnrolmentID.Text = cmbBranchName.Text + "/" + cmbCourse.Text + "/" + cmbYear.Text + "/" + registrationNumber;
  //Registration number is also generated like this (Branch name/Total number of student registered)
  txtRegistrationNumber.Text = cmbBranchName.Text + "/" + registrationNumber;
}

private void cmbYear_SelectedIndexChanged(object sender, EventArgs e)
{
 //enrollment id generated as like <Branch Name/Course/Year/The last registered number according to particular course>
 string registrationNumber = GetLastRegistrationNumberForCourse(cmbCourse.Text);
 txtEnrolmentID.Text = cmbBranchName.Text + "/" + cmbCourse.Text + "/" + cmbYear.Text + "/" + GetLastRegistrationNumberForCourse(cmbCourse.Text);
 //Registration number is also generated like this (Branch name/Total number of student registered)
 txtRegistrationNumber.Text = cmbBranchName.Text + "/" + registrationNumber;
}

Select Max(RegNo)+1 from <tableName> Where CourseName= course

private string GetLastRegistrationNumberForCourse(string course)
{
 string registrationNumber = string.Empty;
 // create connection, command, datareader etc and get the last registration number by 
 // passing the course name or id
 SqlConnection con= new SqlConnection("your connection string");
 SqlCommand cmd= new SqlCommand("Select Max(RegNo)+1 from <tableName> Where CourseName='"+ course+"'",con);
 con.Open();
 registrationNumber = Convert.ToString(cmd.ExecuteScalar());
 return string.Format(registrationNumber, ("0000"));
 con.Close();
}