Cup 3 will fill first, according to this image, because it is connected to Cup 1, which has a lower opening. Though it appears like Cup 4 would fill first, a closer inspection reveals that the pipe from 3 to 4 has no opening at all. Some may say Cup 7, however this is incorrect because the pipe to Cup 2 is at the top end of Cup 1, and Cup 7 has a hole.