When all three bulbs are turned on at once, the bulbs flicker, but then the fuse blows and all the lights go out. This is because during the brief period of "flickering," the current in the main line was 1.2 amps + 1.2 amps + 1.2 amps = 3.6 amps. The 3.6-amp current melts the wire in the 3-amp fuse.