Firstly - there are no slaves in the United States, it no longer matters.
Secondly, the 3/5ths Compromise reduced the power slave states had in the United States federally. If slave states could count slaves proportionally, they would have increased their power in The House of Representatives, while at the same time denying the vote to those people they are representing. What ? We need two more reps to sway a vote in our favor ? Lets bring in a crapton more slaves !
(BTW, this is what Cali is doing with the illegals.. But you don't wanna discuss that, do you ?)