As long as you three vectors all start witht heir tails in the origin, then their sum cannot be zero. BUT sometimes teachers are sloppy in their representations of vectors: It is possible to draw three vectos in the first quadrant ofa x-y graph such that their sum is zero (they would form a closed triangle with all arrows pointing either clockwise or ccw); but this would be due to a misrepresentation of vectors in a quadrant system.