The invention relates to a three-arm tube system (11) for a ventilation appliance (13) suitable for invasive ventilation. Such a ventilation appliance comprises an active inhalation valve and an active exhalation valve (29). The tube system (11) comprises an inhalation tube arm (17), an exhalation tube arm (27) and a ventilation tube arm (37) for attachment to a mask (57), and also a flow sensor (23). According to the invention, a defined leakage (39) is established in the ventilation tube (37), and the flow sensor (23) is arranged between leakage (39) and mask (57). The invention further relates to a ventilation appliance having such a tube system, and to a method for operating a ventilation appliance.
