We present a simple theory of the tuning behavior of a laser resonator with three mirrors, where one pair of acts as a passive etalon reflector for the active main cavity. We are particularly interested in the case mirrors where the active medium is a high-pressure waveguide CO2 discharge and the etalon is an efficient composite reflector formed by a mirror–grating or mirror–lens–grating combination. We show that for this type of device continuous tuning ranges well above the main cavity free spectral range may be possible.