Out of the four speakers i only three are working

Take one of the working speakers and hook it to the output that is not working, this will determine if its a bad speaker or not. if it works then you know the speaker or wiring is at fault. also take the speaker and the wires from the one that isn't working and hook it to a good output and make sure that speaker and its wiring are good. if the output is bad the problem is within the amplifier system, could be a switch, could be output transistors, but now your going to be into money for an estimate from a repair center, i would suggest a local shop, unless this is a new and under warranty system, in which case you should take or send it to the for warranty repair, good luck.