This is definitely a rookie confusion when you are using PlayStation Console or Controller for the first time. Let’s say you are saying a game, maybe “The Last of Us” and suddenly you get a guideline that you need to press L3 or R3 to pick up something or open a door. But in the controller, you only have L1/L2 and R1/R2 physical buttons. It’s very basic and even I got confused when I played for the first time.
L3 - Left stick/L3 button. Press down on the stick to use it as the L3 button.
R3 - Right stick/R3 button. Press down on the stick to use it as the R3 button.
That’s all, no big deal. Let us know if you have any other questions. Thank you!