To form a triangle the sum of the two shorter sides must be greater than the longest side. 5, 5, 3 forms a triangle (isosceles) 8, 8, 8 forms a triangle (equilateral) 5, 6, 10 forms a triangle (obtuse angled scalene)
7, 8, 15 technically forms a triangle with an area of zero units. The vertices would be co-linear. This will often be ignored as it has little practical use.
Fun fact. If you break a stick into 3 random lengths there is only a 25% chance they will be able to form a triangle.
