В Form1:
--------
private void Button_Click(object sender, RoutedEventArgs e)
{
  Form2 dlg = new Form2();
  dlg.Owner = this;

  Hide();
  dlg.ShowDialog();
  Show();
}


В Form2:
--------
private void Button_Click(object sender, RoutedEventArgs e)
{
  Form3 dlg = new Form3();
  dlg.Owner = this;

  Hide();
  dlg.ShowDialog();
  Show();
}

В Form3:
--------
private void Button_Click(object sender, RoutedEventArgs e)
{
  DialogResult=true;
}