The effect of exciting each of the three classes of intermolecular vibrations on the hydrogen bond lifetime (tau (H)) of the isolated water trimer is investigated by far-infrared laser spectroscopy. Single excitation of a librational vibration decreases tau (H) by 3 orders of magnitude to tau (H) = 1-6 ps, comparable to the time scale of a number of important bulk water dynamical relaxation processes. In contrast, excitation of translational or torsional vibrations has no significant effect (tau (H) = 1-2 ns). Although such a dependence of tau (H) on intermolecular motions has also been proposed for liquid water via computer simulations, these are the first experiments that provide a detailed molecular picture of the respective motions without extensive interpretation.