May be you might want to check the PS3 driver that you used; using Xbox360 controller on PC the LB+R3 works fine for virtual mouse, so couldn't be the game itself.

p/s: Bear in mind, most of the PS3 drivers are trying to emulate Xbox360 controller, so could be a translation error in between. If you can, try get/borrow a PC game controller - one that is XInput compatible - to see if it resolves the issue.