public class MainFrame extends JFrame{

...
public MainFrame (){
...
label1.addMouseListener(new PicActionListener());
label2.addMouseListener(new PicActionListener());
label3.addMouseListener(new PicActionListener());
}
 private class PicActionListener implements MouseListener {

        @Override
        public void mouseClicked(MouseEvent e) {
        if(e.getSource().equals(label1)){
           setVisible(false); 
         if (theory == null) {	
            theory = new TheoryNote(this);
    	}
        if (!theory.isVisible()) {
            theory.setVisible(true);  }

        }
       if(e.getSource().equals(label2)){
               //  КОД
         }
       if(e.getSource().equals(label3)){
             //  КОД
           }
}

}

public class MainForm extends javax.swing.JFrame {
    private MainForm mf;
    
    public MainForm(){
        this.mf=this;
    }
}

PicActionListener au= new PicActionListener();
pNote.addMouseListener(au);
tKl.addMouseListener(au);
pKl.addMouseListener(au);
back.addMouseListener(au);