how do you justify if three lenghs make a triangle?
you know that any side of a triangle must be less than the sum of the other two sides.
Consider if two sides are 3 and 5. If you lay them out flat, then the sum is 8. If the third side is more than 8, you can't make the ends meet. 8 is as long as the third side can be, and then it's just a flat line, not a triangle.