用java编写创建三个线程，每个线程应当显示它从开始到目前为止累计运行时间（可以考虑使用Date类或Calendar类）

可以使用Java中的Thread类来创建线程，使用System.currentTimeMillis()方法获取当前时间戳，然后进行时间的计算。以下是示例代码：

import java.util.Calendar;
import java.util.Date;

public class ThreadDemo {
    public static void main(String[] args) {
        Thread t1 = new Thread(new MyThread("Thread 1"));
        Thread t2 = new Thread(new MyThread("Thread 2"));
        Thread t3 = new Thread(new MyThread("Thread 3"));
        t1.start();
        t2.start();
        t3.start();
    }
}

class MyThread implements Runnable {
    private String name;
    private long startTime;

    public MyThread(String name) {
        this.name = name;
        this.startTime = System.currentTimeMillis();
    }

    @Override
    public void run() {
        while (true) {
            long currentTime = System.currentTimeMillis();
            long timeDiff = currentTime - startTime;
            Date date = new Date(timeDiff);
            Calendar calendar = Calendar.getInstance();
            calendar.setTime(date);
            int hour = calendar.get(Calendar.HOUR);
            int minute = calendar.get(Calendar.MINUTE);
            int second = calendar.get(Calendar.SECOND);
            System.out.println(name + " has been running for " + hour + " hour(s), " + minute + " minute(s), " + second + " second(s).");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

上述代码中，我们创建了一个MyThread类实现了Runnable接口，在run()方法中不断计算线程运行时间，并打印出来。在ThreadDemo类中，我们创建了三个线程，分别启动它们。运行程序后，我们可以看到每个线程不断打印出它们的运行时间。