Final answer:
Prescriptions in medicine are written using specific units to ensure accuracy. For a prescription of 0.3 mg, it should be written as "0.3 mg".
Step-by-step explanation:
In medicine, prescriptions are written using specific units to ensure accuracy and avoid confusion. In the case of a prescription for three tenths of a milligram (mg), it should be written as "0.3 mg". This clearly indicates the quantity of the medication being prescribed.