How do you get three players on left 4 dead

On one system? You can't. You can only play co-op between two people on one console because the split screen can't split up into three and still give you visibility you need to shoot the zombies. In order to play with three or four players on Left 4 Dead, two people can be on one console and the third and fourth can be on another or you can all be on separate consoles.