GridLayout，不是GridBagLayout

对于三个大小相等的小部件，使用GridLayout而不是GridBagLayout。请参见Oracle教程How to Use GridLayout。
下面是一个带有两个标签的按钮的示例，所有标签的大小都相同。

package work.basil.example.swing;

import javax.swing.*;
import java.awt.*;

// An abbreviated version of example code provided by Oracle from:
// https://docs.oracle.com/javase/tutorial/displayCode.html?code=https://docs.oracle.com/javase/tutorial/uiswing/examples/layout/GridBagLayoutDemoProject/src/layout/GridBagLayoutDemo.java
public class ThreeWidgets
{
    private static void createAndShowGUI ( )
    {
        JFrame.setDefaultLookAndFeelDecorated( true );

        // Establish the window, represented by a `JFrame` object.
        JFrame frame = new JFrame( "Three Widgets App" );
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );

        makeContent( frame.getContentPane() );

        // Show window.
        frame.pack();
        frame.setVisible( true );
    }

    private static void makeContent ( Container container )
    {
        container.setLayout( new GridLayout() );

        JButton widget1 = new JButton( "Widget 1" );

        JLabel widget2 = new JLabel( "Widget 2" );
        widget2.setBorder( BorderFactory.createLineBorder( Color.GREEN ) );

        JLabel widget3 = new JLabel( "Widget 3 with long title" );
        widget3.setBorder( BorderFactory.createLineBorder( Color.MAGENTA ) );

        container.add( widget1 );
        container.add( widget2 );
        container.add( widget3 );
    }

    public static void main ( String[] args )
    {
        // Assign a task to event-dispatching thread: Create and show our GUI.
        SwingUtilities.invokeLater( ThreeWidgets :: createAndShowGUI );
    }
}