final Handler handler = new Handler();
handler.postDelayed(new Runnable() {
    @Override
    public void run() {
        doSomething();
        count++;
        if (count <= 3) handler.postDelayed(this, 3000);
    }
}, 3000);

myApi
            .getRequestBody()
            .repeatWhen(h-> h.delay(3000L, TimeUnit.MILLISECONDS))
            .takeUntil(stopPolling -> count < 3)
            .subscribe(this::showList);

final Handler h = new Handler();
for (int i=1;i<numberOfTimes+1;i++) {
   h.postDelayed(new Runnable() {
        @Override
        public void run() {
            function();
        }
   },i*3000);
}

public static void main(String[] args) {
    new Thread() {
        @Override
        public void run() {
            while (true) {
                for (int i = 0; i < 3; i++) {
                    ourMethod();
                }

                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }.start();
}

public static void ourMethod() {
    //do smth
}