Limit checks in DataGridView CheckBox

I have a DataGridView with CheckBox, now my question is how do I set a limit on how many CheckBox can be checked into say like 3? I already have the code for counting how many CheckBox is checked. I am new to programming and sorry for my bad english.

private void DataGridView1_CellValueChanged(object sender, DataGridViewCellEventArgs e)
{
        bool isChecked = Convert.ToBoolean(DataGridView1.Rows[DataGridView1.CurrentCell.RowIndex].Cells[0].Value.ToString());

        if (isChecked)
        {
            num += 1;
        }
        else
        {
            num -= 1;
        }
        MessageBox.Show(num.ToString());
    }