The response of three coupled FitzHugh-Nagumo neurons, under high-frequency driving, to a subthreshold low-frequency signal is investigated. We show that an optimal amplitude of the high-frequency driving enhances the response of coupled excited neurons to a subthreshold low-frequency input, and the chemical synaptic coupling is more efficient than the well-known electrical coupling (gap junction), especially when the coupled neurons are near the canard regime, for local signal input, i.e., only one of the three neurons is subject to a low-frequency signal. The influence of additive noise and the interplay between vibrational and stochastic resonance are also analyzed.