All over the moon and it appears to be on many planetary moons in our solar system. The moon's alone is a vast quantity. You are not seriously doubting the presence of He3 there are you?
Regarding payloads dropped to Earth:the energy needed to manuever a payload from Moon to earth is far less than the reverse.
But this is relative; the measure to be judged is the cost compared to the gain. The He3 is extremely valuable. The less costs involved with obtaining the fuel, the better. A Moon base would lower that cost in the long run, considerably.
Two things:
One. What use is He3 btw. I understand that it has some possible uses in thermonuclear weaponry, but other than that what?
Two. If it's all over the moon how do you gather it up? There are tons of gold in a cubic mile of sea water, but no one extracts gold from sea water because it costs too much to concentrate it, and that's right here on earth where you don't have to pay $50,000 a lb to get the machinery in place. Remember the Appolo mission sent back egregiously expensive rocks because of the cost involved in obtaining them, not anything of intrinsic value.
There ain't nothing in space that is worth the cost of getting it.