Well, one mole is equal to Avogadro's Number, or 6.02210 x 10^23 particles. Since you are not talking about the amount of oxygen atoms in 3 moles of O2, no extra multiplication is needed to compensate for that. 3 moles x (6.02210 x 10^23) O2 molecules per mole = 1.80663 x 10^24 O2 molecules.