No one knows really what happened to these three desgnations. AFAIK, there are no other current projects at Airbus with the designations A350/A360/A370. Here is an explanation as to why the went for 380 (although I am not sure it holds that much water...):
350 - Not much a jump from previous technology (A340)
360 - Signifies a fulol circle, whereas the Super-Jumbo is something all-new, not a rehash
370 - 7 is an unlucky number in Chinese/Japanese languages (this doesnt address Boeing's lack of lack of sales for its 7x7 series)
Thus they chose 380, which has the following connotations:
8 is a lucky Asian number
80 is twice 40, A380 carries almost twice as much as A340
8 represents the double deck configuration.
Thats whats been written, anyway, but I doubt its validity...
"So powerful is the light of unity that it can illuminate the whole earth." - Bahá'u'lláh