When I fly straight forward (or backwards) my Phantom 3 deviates left or right slightly.  This is on a calm day with very liitle or no wind at about 2-3 meter height.  Does anything cause this ?  Can I rectify?

I am new to flying and if the concensus is me being an amature pilot so be it but I've done a few flights now and I'm sure when I first started it was OK.

Cheers

Have you done an IMU calibration?  If not, use a completely level surface for your quad (most important) and execute an IMU calibration.  After that do a controller calibration and before flight make sure you do a compass calibration.  

also, ensure you are NOT putting a little and I mean a little pressure on the right stick. check the telemetry in the GO app and see if it is recording a slight stick move....

also ensure you calibrate your controller. (that happened to me when I first got my bird)

More than likely, a slight misalignment of the camera gimbal azimuth.  Mine does this, as do at least a few others.  I've seen several videos with the same problem demonstrated.  I'm attempting to realign the azimuth of the gimbal arm by way of the set screw, but it is not the easiest with the flat shaft.  Still working on it.  

AG0N-Gary Posted at 2016-1-16 12:43
More than likely, a slight misalignment of the camera gimbal azimuth.  Mine does this, as do at leas ...

Hi Gary
And where is this screw and what is the reference point to say its aligned?

Cheers

Look at the upper arm of the gimbal.  It rotates (azimuth) on a shaft that comes out of the bottom camera mount that is suspended under the quad.  That arm is fastened to the vertical shaft and held in place with a set screw.  You can loosen and rotate the camera on that shaft a degree or two and retighten.  It will want to center on the flat side of the D shaped shaft.  You can make it tighten a degree or two off of the flat in the direction to correct the aiming.  It's tricky, and may take a few trial and error times to figure it out.

Thanks all of you for your help.  Followed nrgwise advice about calibrating the IMU and it seems to have solved the problem.  I'd already done the other two calibrations