Leave the tabIndex alone - it only affects the order in which controls are "visited" when the user presses the TAB key.
In the Click event handlers:
private void B1_Click(object sender, EventArgs ew)
{
B1.Enabled = false;
B2.Enabled = true;
B3.Enabled = false;
B2.Focus();
}
private void B2_Click(object sender, EventArgs ew)
{
B1.Enabled = false;
B2.Enabled = false;
B3.Enabled = true;
B3.Focus();
}
private void B3_Click(object sender, EventArgs ew)
{
B1.Enabled = true;
B2.Enabled = false;
B3.Enabled = false;
B1.Focus();
}