-
They need a minimum of 4, because you also need to establish the time. A-GPS is partly about giving a sensible starting location (helpful for the numerical solution of the location problem) but also about caching the almanac (ie satellite locations) in a way that is faster to access. The alternative is receiving it from the satellites directly, but this is slow (many minutes).
(Not that this is really related to your actual point.)
Isn't it the cell triangulation that does this rather than GPS
In movies you are meant to snap your phone in half and throw it in a river, but I think the key element is really the sim card