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This paper pursues RFID support for localization, aiming to pinpoint an object in 3D space. Given a set of RFID tags and/or readers deployed as reference points at known locations in a hexahedron (like shipping container or storage room), a passive and an active localization schemes are considered in this paper. Being the very first range-free 3D localization, our schemes depend solely on RFID tags and readers without other devices or sensors, and it avoids the need of distance estimation according to received wireless signal strength or phase difference. Our passive scheme locates an RFID tag attached to the target object, with both tags and readers as reference points. The active scheme locates an RFID reader, by iteratively determining a 3D sphere best covering the activated reference tags, referred to as the decision boundary optimization scheme (DeB). Results by simulations and testbed experiments using Alien RFID kits have been obtained, and they reveal that DeB outperforms its passive counterpart and achieves the localization error of 0.07 ft. Additionally, DeB yields better location accuracy and yet is much faster than a previous counterpart. With enhanced DeB (EDeB), accuracy of an object located near a hexahedron side or corner is improved considerably. [ABSTRACT FROM PUBLISHER]