Well, I have been playing FPS games with a K&M and I am one happy camper.
I as well moved away from PC so that I stop spending time and money playing catchup, it was an obsession for me so I was so glad I moved away after all those years.
On my PS3, I have played and finished all of the Orange Box (HL2, EP1, EP2, TF2, Portal) , COD MW, BF 1943 and much more with a K&M.
The Solution:
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There are many ways to go about this:
* Your first option would be to get an XFPS Rateup / MAX Shooter PS3 adapter (pick the cheapest you can find). You'll find those online and mainly on Ebay, expect to pay ~$65 for a Maxshooter one and $80+ for an XFPS one
* Second option, is what I use. It's different.
My solution is way before the XFPS rateup adapter came about. Years ago, I devised my own makeshift solution. I used a SmartJoy Frag with a PS-Controller to USB adapter. I had to make some minor modifications to the circuitry of the SmartJoy frag adapter to allow it to give more current to my optical mouse (especially optical ones) using my electronics know how, I bypassed a transistor and that fixed the problem.
Also, you have to make sure you have a PS-USB adapter that supports the PS3's controller number allocation (usually the way you can tell, is that such PS-USB adapters have 1 through 4 LEDs on them just like a PS3 controller. The ones that don't, don't have LEDs 1 through 4 on them)
So there you have it. Nowadays there are plenty of people who figured this out and you can see many videos on youtube. SmartJoy Frag are rare to find and not always for a good price.
I suggest you go with the XFPS, it might save you some extra work, but I have never tried one, but from the videos I can tell it works.