Then there is the word for season 'misseri' which lasts for six months, giving the Norse two seasons-summer and winter only much like us Canadians. In the plural, however misseri means a twelve month period. So, you could make an argument for an eighteen month winter which would be terrible enough but it could very well be three years. Snorri stated in Gylfaginning that the fimbulwinter would be preceded by three winters (years) of fighting. Then a wolf swallows the sun, after which it won't just be cold but dark, too, since another wolf swallows the moon. The gods will be fighting in the dark. Or by the flame of Surt's sword, which everyone would want to crowd around being the only thing giving off heat for a while.
Anyway, since Snorrie mentions the other three years while discussing fimbulwinter, I think he is saying it lasts for three years.