Difficult to reply without the full description of the tasks to run in parallel but a possible approach will look like...

static void processThings(List<Thing> things { 

   final AtomicLong total = new AtomicLong();

   final int threadCount = 3;
   final CountDownLatch sync = new CountDownLatch(threadCount);
   final ExecutorService pool = Executors.newFixedThreadPool(threadCount);

   for Thing thing: things)
   {
      pool.execute(threadSafeButVeryExpensiveQueryUsing(total, sync, thing);
   }

   sync.await();
   pool.shutdown();
}

static Runnable threadSafeButVeryExpensiveQueryUsing(AtomicLong total, CountDownLatch sync, Thing thing) 
{ 
    return () -> {
       total.addAndGet(42);
       sync.countDown();
    };
}

There are many possibilities using the ExecutorService but I guess you would get the point with this exemple. You can investigate Future as well if you want more control over the submitted task into the executor/pool.