How many melatonin 300 mcg would you have to take for 3 mg?

Wrong. They are not equal. 300 mcg equals .3 mg.

And if the questioner is like me, he/she is trying to figure out the difference b/w the two options most available at the store.

They are equal.

It's none of my business, but if you can't convert the units, maybe you should only take exactly what was recommended, so at least you can accurately describe what you put in your body in case something goes wrong.