There is no multiplexer available to do what you wish to do. You can only have 3 sensors (or 2 sensors and 4 touch). However you can use a HiTechnic prototype board (
http://hitechnic.com) and with the right circuit you could develop your own mutliplexer. You could also hook up light sensors to it (any sensor that uses a low amount of current (a single NXT port only supplies so much current and you cant use external batteries)).
What are you wishing to use the 3 light sensors for? If you want to make an array of light sensors for a line follower you should know that it's very easy (and sometimes even more efficient) to make a line tracker with a single light sensor.