3G is a kind of mobile-phone network, short for third generation. It was the generation that made internet and video on a phone feel normal, after slower 2G. Older textbooks still call this “fast”. Though actually, that praise is out of date: in Taiwan most people moved on to 4G and then 5G, and 3G now feels slow; many places have already switched it off. You will still meet 3G on very old phones, in old ads, and in writing that has not been updated. It is a tech label, not a school subject word, but everyone who used a phone in the 2000s and early 2010s met it.