On a PIC, I used its own comparators as a crude multiplexer, but it needs a few more PIC pins to do this, but few external parts are needed. How fast do you want to go? And do you need three full duplex ports? And what, exactly, do you mean by serial devices? A device like a PC or RS-232 type peripheral? Because I2C is a serial interface that you can use to connect many devices to one serial port.