A3-5 refers to a situation or action that is exceptionally reckless or absurd, often leading to unexpected consequences.
Did you see Tom trying to skateboard down that steep hill? Man, that was A3-5! He ended up crashing into a parked car
Used by many groups, not only by Polish speakers, it is mainly utilized to describe someone and their unbelievably foolish actions.
"Bro this dude is such a debil, look at him. He doesn't even know what he's doing"