You'll need to add alternate buttons as well, the PS3 controller is a beast by comparison to the 360 one as far as button layout goes and there is no real line to drive high or low like on the CG board to make a button/trigger press. This is what throws the wrench in the works for using the actual triggers as the rapid fire buttons, something that can easily be done on the 360 controllers because of how they're setup, the PS3 is not one thing like them at all.
It could probably be done if ya got in there and severed the carbon film on the daughter board's button contact so the circuit was open and ya had some way to actually detect the trigger was pulled. This is only a guess and would technically change the way the controller would work in the 'default' mode and is something I haven't even thought of trying until right now. The way it is now it's all a closed setup and just a right mess, so doing it anywhere near like how the 360 controllers can be done isn't gonna happen easily.
In order to do it right now you'll be using a PIC, 4066 IC and an alternate button for every button on the controller ya want to be a R/F one. This controller is just so far from what and how the 360 ones work it's not even funny really, ever since the DualShock 2 controllers came out they've been a mess.