Hi- Brand new to this. I have a new Phantom 2 (V3) H3 on trying to calibrate compass, the lights go green on the 360 horizontal deg turn, but when I do the vertical 360 turn I consistently get  a slow red blink - tried numerous times, always get the red blink after the second action? Any advise me why please? Cannot find in any manual an answer. thx in advance

If you are in NAZA mode, its normal.
After the final turn the LED's go back to displaying flight mode, so NAZA is Red / Green or Phantom mode is Yellow / Green.
The manual does explain this...

Make sure you have no magnetic or ferromagnetic ( metal )  interference in the vicinity where you do your calibration at ( that includes things that you carry on your body )

Most probably your battery is low.
If the Compass calibration fails you get red yellow flashing.  If it succeeds you get whatever is normal for the current situation.  Slow Red means battery is low but not critical.

Also note that calibration failure is indicated as red - Yellow flashes of the LEDs