...
b1.addActionListener(this);
...

public void actionPerformed(ActionEvent e) {
        String command = e.getActionCommand();
                  
        if (command == DISABLE) { //They clicked "Disable middle button"
            b2.setEnabled(false);
            b1.setEnabled(false);
            b3.setEnabled(true);
        } else { //They clicked "Enable middle button"
            b2.setEnabled(true);
            b1.setEnabled(true);
            b3.setEnabled(false);
        }
    }


full code listing