3 legged chairs don't rock but the legs need to stick out much further to give stability against tipping over. 4 legs don't need to stick out as far and 5 legs can stick out less again,
The Alexander Kielland was a 'penta' semisub; i.e. it floated on five circular pontoons. When one leg support failed it capsized with the loss of 123 lives.
https://en.wikipedia.org/wiki/Alexan...and_(platform)