Low-power techniques are proposed for the wireless three-dimensional Network-on-Chips (wireless 3-D NoCs), in which routers on the same chip are connected with metal wires while those on the different chips are connected wirelessly using the inductive-coupling. For saving power consumption of the vertical link, the clock and power supplies to the transmitter are stopped when their utilizations are between a specified range. Meanwhile, the whole wireless vertical link will be shut down when the utilization is lower than the threshold. In order to keep performance, on-demand activation is used in this paper. As long as flit comes, the dormant data transmitter or the whole vertical link will be activated immediately without any judgement. Full-system many-core simulations using power parameters derived from a real chip implementation show that the proposed low-power techniques reduce the power consumption by 23.4%-29.3%, while the performance overhead is less than 2.4%.