Do you really mean a multiplexor or do you mean an encoder.
A binary 8 to 3 line encoder is very simple.
If you mean a multiplexor, what is being multiplexed? Do you mean eight separate inputs, each going to one of three outputs with the output for each input decided by three address select bits. If that is the case, the routing logic is straight forward but preventing a collision (more than one input connected to a single output) is significantly more complicated.
Brian.