We're not asking you to find the area of this triangle, so you're using the wrong tool for the job ;-)

vectors are zero-indexed, look at your loop closely ;)

1. You can cout the input to see what you receive.
2. Your Heron's formula is wrong, here's a correct one:

p = (a + b + c) / 2
S = sqrt( p * (p-a) * (p-b) * (p-c) )

3. You're overcomplicating it for yourself (there's a certain very simple rule which let's you know if a, b, and c form a triangle).