How can I compute resultant velocity vector and magnitude from a 3D time-series of water flow?

I have a number of 3D velocity time-series each with 1500 simultaneous data points in 3D (i.e. VelX, VelY, VelZ).

Is there an existing function that will rotate the time-series and provide me with a resultant vector and magnitude?

[Martin Wilkes]

To clarify. I want to rotate the data so that the ensemble averages of VelY=VelZ=0.