I can't exactly remember what happened when I used them. But can you change the display settings on the PS3 to 720p or anything like that?
If you can't then your problem is right there, you need Component Cables compatible with your PS3, not Composites.
If your TV is not a HDTV and doesn't support Component cables, then your stuck with the Composites and may have to buy a HDTV.
You may not have to buy a HDTV though. Browse on Google or somewhere to see if you can find out how to "stretch" the frame if you know what I mean in an editing program, though this may result in poorer quality.