For RX signal of PIC, you will need a 3-to-1 multiplexer. For TX signal of PIC, you will need a 1-to-3 demultiplexer.
Have a look at the following links:
http://www.play-hookey.com/digital/m...our_input.html
http://www.play-hookey.com/digital/d...emux_four.html
Remember that you would need 3 separate level shifters (MAX232, for example) in total, when you use commercial multiplexer/demultiplexer chips (because they are logic chips).
Sorry I don't have time to create a schematic for you!
Cheers,