If millikan was measuring the charge on an oil droplet with 3 negatively charged electrons on it, what charge would he have measured on the droplet?

The unit of measurement for charge is Coulomb. Now, every particle of electron contains a charge equal to 1.602*10⁻¹⁹ C. So if there are 3 electrons in the oil droplet, then the total charge would be:

Total charge = 3 electrons (1.602*10⁻¹⁹ C / electron)

Total charge = 4.806*10⁻¹⁹ C