The first answer is essentially correct, but if you want the very simplest way to accomplish the desired result, here's a stand-alone code example demonstrating how to do it with a semaphore (which is also how dispatch groups work behind the scenes, JFYI):
#include <dispatch/dispatch.h>
#include <stdio.h>
main()
{
dispatch_queue_t myQ = dispatch_queue_create("my.conQ", DISPATCH_QUEUE_CONCURRENT);
dispatch_semaphore_t mySem = dispatch_semaphore_create(0);
dispatch_async(myQ, ^{ printf("Hi I'm block one!\n"); sleep(2); dispatch_semaphore_signal(mySem);});
dispatch_async(myQ, ^{ printf("Hi I'm block two!\n"); sleep(4); dispatch_semaphore_signal(mySem);});
dispatch_async(myQ, ^{ dispatch_semaphore_wait(mySem, DISPATCH_TIME_FOREVER); printf("Hi, I'm the final block!\n"); });
dispatch_main();
}