Step-by-step explanation:
An atom is the smallest building block of all matter that exists singly. Like sodium atoms.
A molecule could be either a group of atoms of the same element, like oxygen (O2) OR a group of atoms of different elements like all compounds (eg. carbon dioxide, CO2).
A compound is a molecule made of atoms of different elements joined by chemical bonds. Like water (H2O), table salt (NaCl).