That's a common fallacy about the "Three-fifths" clause. That wasn't something put in the Constitution by slaveowners who considered blacks only 3/5 human. It was inserted by abolitionists! It was done to reduce the effectiive populations of slave-holding states in the South, and thereby reduce their Congressional representation, so that it would be MORE difficult to passs laws allowing slavery.
Of course, the idea that all those "old dead white men" could have had blacks' interests in mind doesn't play well with the Al Sharptons and Jesse Jacksons of the world, so it's more useful for them to say the 3/5 clause was just another slap in the face to blacks.