String[] comboBoxItems  = from DB

         for  ( int  i= 0 ; i<comboBoxItems.length; i++)
        {
          jComboBox1.addItem(comboBoxItems[i]);
        }

* JComboBox(model), e.g.: 

String query  =  "SELECT <column> FROM <table>"; 

PreparedStatement stmt = con.prepareStatement(query); 
ResultSet         rs   = stmt.executeQuery(); 

DefaultComboBoxModel comboModel =  new  DefaultComboBoxModel(); 

 while  (rs.next()) {  comboModel.addElement(rs.getString( 1 )); } 

JComboBox combo =  new  JComboBox(comboModel); 

/*	 * ComboBoxModel from Database	 */

 public   class  oraJCDM  extends  DefaultComboBoxModel
{
   private  Vector items =  null ;

   public  oraJCDM ()
  {
	 super ();
	 this .items =  new  Vector();
    
          // Здесь зачитка даных и тд...

           while  (rs.next()) 
          {                  
             for  ( int  i= 1 ; i<=numberOfColumns; i++) 
            {
              items.addElement(rs.getString(i));
            }
            row++;              
          }
          rs.close();

    
    
	Iterator it = items.iterator();
	 while  (it.hasNext())
    {
	 super .addElement(it.next().toString());
    }
    
  }

   private   void  jButton_actionPerformed(ActionEvent e)
  {   
    oraJCDM model =  new  oraJCDM();
    jComboBox1.setModel(model);
  }

Может и так... но проще

    
oraJCDM model =  new  oraJCDM();
jComboBox1.setModel(model);

    
oraJCDM model =  new  oraJCDM();
jComboBox1.setModel(model);
jComboBox2setModel(model);
jComboBox3setModel(model);