public void OnChanged(object source, FileSystemEventArgs e)
{
    i++;
    var data = File.ReadAllLines(e.FullPath);
    string last = data[data.Length - 1];

    if (i == 1)
    {
        tb_art1.Invoke(new Action(() => tb_art1.Text = last));
    }
    if (i == 2)
    {
        tb_art2.Invoke(new Action(() => tb_art2.Text = "ART2"));
    }
    if (i == 3)
    {
        tb_art3.Invoke(new Action(() => tb_art3.Text = "ART3"));
        MessageBox.Show("Com?");
        tb_art1.Invoke(new Action(() => tb_art1.Text = ""));
        tb_art2.Invoke(new Action(() => tb_art2.Text = ""));
        tb_art3.Invoke(new Action(() => tb_art3.Text = ""));
        i = 0;
    }