3 successive winters, same bees, same hive, you can start to call them survivor stock. But, IMO, they don[t really qualify until they've survived 5 successive winters. Why? Because winter severity varies greatly (as we've seen this year, especially) and it is not uncommon, in my area at least, to have 3 successive mild winters, but winter 4 or 5 will most likely be a killer, much like this year, lots of snow, lots of cold, wild flucuations in short periods of time.
Bees need to be tested to properly be labeled survivor stock and 5 years provides a more proven period of survivability.