The 3900X was one of the highest-end consumer Zen2 CPUs. If I'm remembering correctly only the 3950X was above it without jumping up to the Threadripper line of CPUs.

Ryzen 3xxx would be the 3rd generation of Zen CPUs. For example, an Ryzen 3200U (U means it has an embedded GPU, basically an APU) would be low-end, Ryzen 3500 would be middle-end, and Ryzen 3700/3900 would be high-end.

The first number signifies the generation. What you mean is intel with its i3/i5/i7/i9 marketing Names to segment features behind arbitrary performance classes. AMD3XXX goes from 4 Cores on 3100 to 3995 with 64 cores.