That is partially true. It is some kind of average among all cases where there is 3 base stations "see" one phone. But it is heavily depends on the used cellular network generation, distance to base stations, landscape, buildings around and the hell ton of other stiuff. In towns where base stations are on every block corner precision is much higher, up to hundreds of square meters, if the network is less dense, reflections from buildings and interference lower the precision. In the rural area when there is only one base station see mobile device it is a 90-120 degrees arc with the width of few hundred meters and length depending on the distance from base station, i.e. from that ¾ square mile close to the tower to dozens of square miles far away from it.

Really, triangulation is not the main source of phone position. GPS receiver in phone often permanently on and it's data constantly shared with multiple parties, from all that shitty social networks to the "innocent" app of some cafes of gas stations installed because of that annoying "do you have our app to get discount and collect bonuses?" each fucking time you visit that place. And of course most people just have navigation app turned on permanently.

That shit share your position with precision of few meters.