Which Melatonin 300 mcg or 3 mg is smaller?

They are the same.

No they are not. 1 milligram is 1/1,000 of a gram (milli = thousandth). 1 microgram = 1/1,000,000 of a gram (micro = millionth). So 300 micrograms is equal to .3 milligrams.

The answer to the question is that the 300 mcg tablet is a smaller dose.