In theory it is not a serious fault if the manouvre was under control and observations safe. However, if the road was wide enough to undertake the manouvre in 3 points, then it is likely to result in a serious fault. A 7 Point turn may be required if the road is very narrow.
Comment by Trevor Moore:
I always say that the examiner takes into account the width of the road and the size of the car. Some cars have excellant turning circles, others don’t. They don’t call it a three point turn anymore because of this. Quick steering to full lock and very slow movement of the car is the key.