C'mon, people. Oh, I mean those of you over the age of 45. (Those under that age probably weren't provided this sort of an education, as self-esteem has replaced information in the acedemic schema, so it's not your fault.) But the rest of you. Did you skip history class that day?
The U.S. Constitution didn't say that a slave was only 3/5's of a person. It gave representative votes in the House to southern states at a reduced rate, so that the southern states weren't disproportionately over-represented in Congress based on the number of people in the state -- when some of them were disenfranchised by that state by reason of being treated as property.
In short, the writers of the U.S.Constitution used the 3/5 fraction to discriminate against slave holding states when it came to votes in Congress.
It was a compromise. The slave states wanted representation based on the number of people in their state, slave and free. The non-slave states wanted representation in Congress based only on the free population of the slave states. A deal was cut, because there wouldn't be a union if the north got their way altogether, because the slavery south just wouldn't ratify the constitution. And vice-versa.
Those of you who are still yelping about this after all these years are either cynically trying to drum up political enthusiasm, or you are showing your ignorance altogether. Or both.