The three vectors form a triangle, but you are by no means guaranteed that it is an equilateral triangle. As for your answer, what kind of an answer are you to compute? I don't think you're going to be able to find the exact number.
Ok, so i have Vectors A + B + C = 0
From this, am i right in thinking that it forms an equilateral triangle? Or is that incorrect?
I need to use the above to find the answer to a.b + b.c + c.a
It seems as if you have simply posted only a piece of a problem.
If you post the complete question, we may be able to help you with it.