The element that will gain three electrons to form an anion is Nitrogen (N).

Here's why:

* Nitrogen's electron configuration: Nitrogen has 5 electrons in its outermost shell.

* Octet rule: Atoms tend to gain, lose, or share electrons to achieve a stable configuration of 8 electrons in their outermost shell (the octet rule).

* Anion formation: To achieve a stable octet, nitrogen needs to gain 3 electrons. This results in a negative charge (-3), forming the nitride anion (N³⁻).