Final answer:
To convert 3 grains to milligrams, use the conversion factor of 1 grain = 65 milligrams.
Step-by-step explanation:
To convert between apothecary, metric, and U.S. Customary systems of measurement, you need to know the conversion factors between the units. In this case, we are converting from grains (gr) to milligrams (mg). Since 1 grain is equal to 65 milligrams, you can use this conversion factor to solve the problem:
3 grains x (65 mg / 1 grain) = 195 mg
Therefore, 3 grains is equal to 195 milligrams.