Here's why:

* Aluminum's electron configuration: Aluminum has an atomic number of 13, meaning it has 13 protons and 13 electrons. Its electron configuration is 1s² 2s² 2p⁶ 3s² 3p¹.

* Valence electrons: Valence electrons are the electrons in the outermost energy level of an atom. For aluminum, the outermost energy level is the 3rd energy level (3s² 3p¹).

* Counting the valence electrons: The 3s sublevel has 2 electrons, and the 3p sublevel has 1 electron. In total, aluminum has 3 valence electrons.