Cup 3 is the first and ONLY cup that will fill. Water from cup 1 will flow into cup 3 before flowing into cup 2 because the outlet pipe to 3 is lower than 2. No water will flow into either 4 or 5 because the outlet from cup 3 to cup 4 is blocked and pipe from 3 to 5 is blocked where the pipe would discharge to 5. When 3 fills up cup 1 will flow into cup 2 but cannot flow into 6 because the pipe outlet from 2 to 6 is blocked. Water will flow into 7 but since 7 has a hole in the bottom it will never fill. Therefore only cup 3 will EVER fill.