The main reason you always hear the '3 year rule' is because it's been that way since 2000 at least.
'00, '00i, '02,
'04, '05, '06
'07, '08, '09
It's mainly prediction based on history. I don't think I ever saw anything 'official'
I have seen employee comments that they run a general 'trend' of changes from year to year... first year is a major internal change which includes a file version update. Second year includes new tools, refined tools. Third year includes interface updates.