1. Consider a triangle with sides a, b and c.
2. a, b and c can be sides of a triangle only if each of these conditions hold:
i) a+b>c
ii) b+c>a
and iii)a+c>b
3. So the only condition for 3 numbers to be the side lengths of a triangle is that the sum of any 2 of them must be larger than the third number
4. This is called the "triangle inequality".