An electronic device may include first, second, and third antennas and conductive housing structures. The first, second, and third antennas may each include slots having open ends defined by gaps in the conductive housing structures. The second antenna may be interposed between the first and third antennas. The first and second antennas may convey signals at the same frequencies. The third antenna may convey signals at a lower frequency than the first and second antennas. A switch may be coupled across the third slot and may have a first state at which the switch forms a closed end of the third slot and a second state at which the third slot has two opposing open ends. Control circuitry may selectively activate one of two feeds for the third antenna and may adjust the switch so that the third antenna exhibits satisfactory antenna efficiency regardless of environmental conditions for the device.