and also we should calculate frame border size to find best main panel
size..

//

*

* Learn how to use layouts, and the problems should be fixed.

Andrew T.

Don't do that. Use a layout manager and let it figure out the available
remaining space. That's what they are for.

/Thomas

Use layouts, they make this sort of thing easy.

import java.awt.BorderLayout;
import java.awt.Dimension;

import javax.swing.BoxLayout;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.border.TitledBorder;

public class TriBox {
    public static void main(String[] args) {
        JFrame frame = new JFrame();

        JPanel p1 = new JPanel();
        p1.add(new JLabel("Panel 1: height fixed at 200 pixels"));
        int width = (int)p1.getPreferredSize().getWidth();
        p1.setPreferredSize(new Dimension(width ,200));
        p1.setBorder(new TitledBorder("1"));

        JPanel p2 = new JPanel();
        p2.add(new JLabel("Panel 2: height fixed at 100 pixels"));
        width = (int)p2.getPreferredSize().getWidth();
        p2.setPreferredSize(new Dimension(width ,100));
        p2.setBorder(new TitledBorder("2"));

        JPanel p3 = new JPanel();
        p3.add(new JLabel("Panel 3: variable height"));
        p3.setBorder(new TitledBorder("3"));

        JPanel fixedPanel = new JPanel();
        fixedPanel.setLayout(
                new BoxLayout(fixedPanel, BoxLayout.PAGE_AXIS));
        fixedPanel.add(p1);
        fixedPanel.add(p2);

        frame.setLayout(new BorderLayout());
        frame.add(fixedPanel, BorderLayout.NORTH);
        frame.add(p3, BorderLayout.CENTER);

        frame.pack();
        frame.setVisible(true);

    }
}