The protonated water trimer is a prototype “proton wire” that can transport two protons nearly concertedly. This “proton transfer mode” (PTM) is an important contributor to the infrared spectrum of the isolated gas-phase cluster. We have simulated its infrared spectrum for both the hydrated and deuterated isotopologues, using vibrational 2nd order perturbation theory (VPT2) and ab-initio molecular dynamics (AIMD) trajectories. VPT2 calculations explain quantitatively the experimental spectra at both high and low frequencies, provided that high-level quantum chemistry is utilized. In the D2D2-tagged hydrated cluster, the PTM undergoes giant Fermi resonances (FR’s) with two combination bands. In the deuterated analogue, one observes a single FR of “normal” intensity, manifested as the doublet recently reported experimentally. We provide band assignment for both isotopologues, with and without the D2D2 tag applied in experiment. We discuss possible manifestations of the giant resonance on proton transfer through water wires.