Answer
Iron (Fe) is an atom.
Hydrogen gas (H$_{2}$) is a molecule.
Ammonia (NH$_{3}$) is both a molecule and a compound.
Work Step by Step
Atoms are the smallest unit of matter that has the properties of a chemical element. Fe is seen here in its smallest form, therefore it's an atom,
Molecules are composed of two or more atoms held together by a chemical bond. The atoms may identical (such as seen in H$_{2}$) or different (as seen in NH$_{3}$).
A compound is a type of molecule that consists of at least two different elements. Therefore NH$_{3}$ is a compound.
