True, the 3/5ths compromise had to do with allocating representatives in the House of Representatives under the Constitution, but original formula of "five slaves equals three free men" dates back to the Confederation period.
Yes, in 1783, buy in in 1787 it was adopted in the Constitutional Convention, and the representation aspect was a far bigger issue then. I see the distinction you make, though, under the Articles of Confederation.
Good point.