What are 3 atoms in each formula unit?

In a formula unit, the three atoms can vary depending on the substance being described. For example, in sodium chloride (NaCl), there are one sodium atom (Na) and one chlorine atom (Cl). In calcium carbonate (CaCO₃), the formula unit consists of one calcium atom (Ca), one carbon atom (C), and three oxygen atoms (O). Another example is water (H₂O), which contains two hydrogen atoms (H) and one oxygen atom (O).