300-400-500 didn't come about as a measure for how good someone is at powerlifting, just happens it's for the same lifts.
It's an old Hardgainer concept. Hardgainer being a long running magazine about bodybuilding and strength training.
The idea is that any hardgainer (someone who's not particularly genetically gifted) can reach a 300lb bench, 400lb squat and 500lb deadlift if they put in enough work. It's something that gets used as a goal by a lot of people.
Obviously for different people and different body types the numbers might be a bit out of whack. My squat is about the same as my deadlift, so I hit the number for the squat way before I did the other two.
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Of course you're not an elite powerlifter if you can only lift that amount in a comp (assuming you're not small). But you're not weak either.
There aren't many people in most gyms who can bench 300 and squat 400 and deadlift 500, even less if you think about ordinary gym goers training for a physique or strength, but not for a powerlifting meet.