Accurate indoor positioning has the potential to transform the way people navigate indoors similar to the way the GPS transformed outdoor navigation. Over the last 20 years, many indoor positioning technologies have been proposed and experimented by both academia and industry. We developed an accurate acoustic 3D positioning system, which consists of one mobile device (MD) and four base stations (BSs) with known locations. The MD transmits a series of ultrasound and RF sequences, named Zadoff-Chu (ZC) sequences, at the same time. The propagation time of the RF signal is negligible compared to the ultrasound signal, due to the large speed of light compared to sound. Therefore, the RF ZC sequences are utilized to synchronize the MD with the BSs. The distance from the MD to each of the four BSs is estimated by estimating the time of flight (ToF) of the transmitted sequence. Finally, these distances are converted to a 3D position using a trilateration algorithm. Our positioning system won the fourth place in Microsoft Indoor Localization competition 2017, in a two-floor area where each floor is 18m x 20m x 4m.