* 3 molecules: This tells us we have three units of the compound.

* Mg: This is the symbol for magnesium, indicating one magnesium atom per molecule.

* O: This is the symbol for oxygen, indicating one oxygen atom per molecule.

* H: This is the symbol for hydrogen, indicating one hydrogen atom per molecule.

Now, let's calculate the total atoms:

* Magnesium: 3 molecules * 1 Mg atom/molecule = 3 Mg atoms

* Oxygen: 3 molecules * 1 O atom/molecule = 3 O atoms

* Hydrogen: 3 molecules * 1 H atom/molecule = 3 H atoms

Therefore, there are 3 magnesium atoms, 3 oxygen atoms, and 3 hydrogen atoms represented by the notation 3 molecules.