The PlayStation 3’s system software does not support per-game button remapping. Control schemes are hardcoded by developers, so you can’t simply reassign R1/L1 to R2/L2 through the console’s settings.
On original PS3 hardware, you’re stuck with the developer’s default layout. Many shooters do offer “Southpaw” or “Legacy” presets in their options menus—check those first before investing in extra hardware.