The Trophy lists being patched into PS1 games via PS Plus Premium have proven very simple up until now, allowing you to have a classic title with 100 per cent completion forever on your PS5, PS4 profile. And Syphon Filter 3, available right now via January 2023's PS Plus drop, is no different. All you need to do is beat the game.
Across 21 digital gongs — which includes the Platinum Trophy — you'll unlock one for beating each level. There are 19 missions in the game, so you'll be unlocking them all at speed, with the title estimated to take around eight hours to beat. The only other thing you need to be aware of is killing an enemy with a knife, which unlocks a Bronze Trophy. It can be unlocked very early on in the game, so it's hardly even worth worrying about. Effectively, just complete Syphon Filter 3 and another Platinum Trophy from a Classic title is yours.