Here's why:

* Valence electrons are the electrons in the outermost shell of an atom, and they are the ones involved in chemical bonding.

* Group number in the periodic table generally corresponds to the number of valence electrons an element has.

* Group 3 elements are located in the third column of the periodic table.

Therefore, Group 3 elements like scandium (Sc), yttrium (Y), and lanthanum (La) all have 3 valence electrons.