Vector sum of Equal 3 phases 120 degree apart is zero and serves as a neutral.However,while drawing instantaneous vectors of the three phases,they summed up all together to give a vector.I have attached the animation which shows: "An ordinary three phased system in both vector form and in sinusoidal form. The black vector is the resultant space vector; a vector sum obtained by adding the three vectors. As can be seen, the space vector's magnitude is always constant".

Kindly Explain the difference between the two approaches.