An electronic device may have a metal housing. The metal housing may have an upper housing in which a component such as a display is mounted and a lower housing in which a component such as a keyboard is mounted. Hinges may be used to mount the upper housing to the lower housing for rotation about a rotational axis. A slot-shaped opening may separate the upper and lower housing. Flexible printed circuits with ground traces may bisect the slot-shaped opening to form three electrically isolated slots each of which is aligned with a respective cavity antenna. The antennas may have antenna grounds formed from portions of the metal housing and other conductive structures. Resonating elements for the antennas may be supported by an elongated ventilation port structure along the rear edge of the lower housing.