Location estimation in indoor/campus environments has attracted much interest for its broad applications. Many applications (e.g. personnel security) require not only the 2-D coordinate but also the floor index where the mobile users are situated. However, most of the current location systems cannot provide the floor information accurately and robustly. In this paper, we propose a 3-D localization scheme which fuses the barometric sensor with Wireless LAN (WLAN) signals and building information. Our experiments show that this fusion scheme can both identify the floor index without errors and improve the horizontal localization accuracy. Moreover, since the barometric sensor is quite simple and cheap, it would bring almost no increase in system costs.