Why A three-terminal regulator such as the 78xx/79xx or LM317 family has an input pin, an output pin, and a common/ground (or adjust) pin. Inside, a pass transistor sits in series between input and output, and an error amplifier compares a sampled fraction of the output against an internal reference to control how hard that pass element conducts. Because the control element carries the load current in series with the load, it is a series regulator, and it only draws as much current as the load needs plus a small quiescent amount.
Watch out A shunt regulator, like a simple Zener or a TL431, is connected across the load and regulates by diverting excess current to ground through a dropping resistor, so it wastes current at light loads and is not how the common three-pin regulator ICs work.
Three pins: in, out, ground. The pass transistor is in line with the load, so it is a SERIES regulator.