The second year civil engineers at my university had a project where they had to design a bridge that could support 2 people, but would break under the weight of 3, to teach that exact lesson.
People vary in weight by more than 50%. I don't see that the spec is possible.
Woosh..
That's the point!
The three people are known to you in advance (it’s your group). It was common to game this a little by saving the biggest person as the third one that had to break the bridge.