One mole in chemistry is defined as the number of unit particles that are contained in 12 g of carbon-12. This is given as 6.02214179(30)*10^23 particles. The mass of one mole of any substance will have the same number of unit particles.
In questions like the one you have given, where you have to find the number of atoms contained in 3 moles of Iron(III) oxide, follow these steps.
First look at the molecular composition of Iron(III) oxide. It is Fe2O3, so one molecule of the iron oxide has 2 atoms of iron and 3 atoms of oxygen.
1 mole of Iron(III) oxide has 2 moles of iron and 3 moles of oxygen.
The number of atoms of iron in 3 moles of Iron(III) oxide is 3*2*6.02214179(30)*10^23 or approximately 36.13285*10^23.