What does a STR of 3 mean in old school games?
Well, first let's talk about AD&D 1st Edition. At the break point of 1E, the Strength stat was for the first time assigned a value.
|Chart Copyright 2000-2002 by Stephen Nispel|
We can see that the STR 3 guy can lift an astonishing 10 pounds over his head! And he has a 0% chance of bending steel bars or lifting a portcullis (clearly two especially important actions in 1st Edition.) In later editions and up until the present day, we have imagined that a STR of 3 correlates to the weakest of the weaklings across the spectrum of people everywhere.
But in old school play, this simply is not so! A Strength score of 3 has no exact weight correlation. A STR of 3 can certainly mean someone who is a complete weakling, but it doesn't make sense that a veteran adventurer would be too feeble to carry his own gear, does it? No, it does not. Instead, a STR of 3 just means your man is very weak compared to other adventurers - perhaps he would have little chance of winning an arm wrestling contest. Certainly he would have little chance to bend iron bars or break down a door with his bare hands, among other feats of strength.
But a STR 3 character is strong enough to carry his gear, wear his armor, and swing a weapon (just perhaps not as forcefully as Conan might do). As far as other strength-related tasks, he would likely fail most of the time. That's all it means.
So therefore what can we say about your man when you roll a 3? Perhaps he is a child. Perhaps he is very old. Perhaps he is as strong as a normal person but so grand that it takes all his might just to propel himself around on his two tree-trunk legs! And this will get me in trouble to say in this day and age, but perhaps he is actually a she* instead? If you really want to get kooky, maybe you say your man only has one arm or his legs do not work. He's unable to perform a wide range of feats of strength because of this drawback.
Do not despair when you roll a 3 for Strength. Let it inform you about your man. You will be all the more heroic for overcoming such a limitation.
*There ought to be no mechanical difference between male and female characters, so you are free to play a woman with an 18 STR. However in the real world almost all women are less strong than almost all men. It's just a fact of biology.