Don't you get the same period of cover no matter when you buy it?
I think the way Applecare works is that you get 3 years from the original date of purchase of the device, so it doesn't matter at what point you buy it during the first year of ownership.
i.e. If you buy Applecare with with machine you get three years cover & if you buy one day before the end of the first year, you get 2 years (and one day), so the cover expires on the same date in both cases.