3 nanometers is generally a marketing number to indicate relative fineness of the process. But it has real relevance to the physical structures that are manufactured by the process.

Transistors have many distinct features. There's the current source and drain, with the gate between them. There's a number of different ways to build these structures.

As a lousy analogy, a light switch has lots of parts: wire terminals, contact points to make a tight connection to a conducting blade that can be moved with a fancy handle, etc.

Maybe you want to print your switch with your home 3D printer. A really fancy one that can print lines that are 3nm in width on a good day.