The easiest way is to use a monitor that has an HDMI port and use an HDMI cable. 
But if your monitor does not have an HDMI port, then you will want to look at what connectors it does have.
If it has a DVI connector, then you can use an HDMI to DVI converter(this removes the audio though, so you'll need to use an audio cable for the PS3 to some sort of speakers).
If the monitor only has VGA(or D-Sub), you will have to look for cables or devices to convert the output from the PS3 to a VGA connector. The only thing I've seen at Futureshop that will work for this is a TV Tuner on your computer. You would connect the PC to the monitor as normal, the TV Tuner to your PC, and the PS3 to your TV Tuner. If you choose this route, I would recommend one such as the ATI TV Wonder HD 650 Combo USB TV Tuner.
