From Wikipedia, stone is defined as a unit：
The stone or stone weight (abbreviation: st.) is an English and imperial unit of weight or mass now equal to 14 avoirdupois pounds (6.35029318 kg).
One stone is 14 pounds; Three stone would be 42 pounds.
A weakling is a person or animal that is physically weak and frail.
What does "three-stone weakling" mean? A weak person whose weights 42 pounds?
There is another phrase about three-stone: three-stone ring. Does "three-stone" in this phrase mean 42 pounds too? It is quite heavy!