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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.)
hamrack
No. GPS uses a fix on three geo-stationary satellites to calculate your position. Most phones support Assisted GPS which gives a quick fix based on cell tower triangulation, but that’s done simply to speed up the users ability to use their location.