The 3/5 clause was a compromise to accomplish the Constitution with the hope that eventually slavery would die.
That is why it was banned in the NorthWest Ordinence and importation of slaves was also banned.
The South's view changed when cotten came into play, and slavery went from an evil that had to be eventually removed to a positive good.
This was admitted by Stevens himself.
But not to be tedious in enumerating the numerous changes for the better, allow me to allude to one other -- though last, not least. The new constitution has put at rest, forever, all the agitating questions relating to our peculiar institution -- African slavery as it exists amongst us -- the proper status of the negro in our form of civilization. This was the immediate cause of the late rupture and present revolution. Jefferson in his forecast, had anticipated this, as the "rock upon which the old Union would split." He was right. What was conjecture with him, is now a realized fact. But whether he fully comprehended the great truth upon which that rock stood and stands, may be doubted. The prevailing ideas entertained by him and most of the leading statesmen at the time of the formation of the old constitution, were that the enslavement of the African was in violation of the laws of nature; that it was wrong in principle, socially, morally, and politically. It was an evil they knew not well how to deal with, but the general opinion of the men of that day was that, somehow or other in the order of Providence, the institution would be evanescent and pass away. This idea, though not incorporated in the constitution, was the prevailing idea at that time.(emphasis added)
By the way, with that statement, Stephens went against the Dred Scott decision, in which Taney stated that the Founding Father's never intended for slavery to be ended.
If you produce some legislation or documentation that the South formally denounced the DOI, I'll stand down. Until then, I consider your claim inadmissible.