Question

on the map, #3 is identifying which mountain system?

Explanation:

The map shows mountain systems in the US. The #3 is in the eastern US, which corresponds to the Appalachian Mountains. The Appalachian Mountain system is a major mountain range in the eastern United States, characterized by its location and typical mountain - range representation on such maps.

Answer:

The mountain system identified by #3 is the Appalachian Mountains.