Take the three sides of a triangle, add them in pairs. Check if the sum is greater than (not equal to) the third side.
If that is true, then it is possible to construct a triangle with the given side lengths.
Let us try the measure of line segments given above.
3 + 5 > 3 3 + 3 > 3 5 + 3 > 3
8 > 3 6 > 3 8 > 3
You see, each side lengths are greater than the third side length, therefore, you can construct a triangle out of these segment lengths.
