Apparent power definition
Apparent power is the vector sum of real power and reactive power. In effect it is V\$_{RMS} \times I_{RMS}\$ without any consideration of phase angle. Because real and reactive power are at right angles, the vector sum is: -

\$\sqrt{Real^2 + Reactive^2}\$

Once you have it for one phase then it you can, just like in real power) multiply by 3 to get the value for a balanced supply/load 3-phase system.