At the time of the 3/5 Compromise, slaves were not allowed to vote. Neither were women or those who did not hold property. Thus the human beings were counted for population numbers only. Since slaves were not free to vote nor would they likely to be so, and their numbers so preponderant that the southern states would have an overwhelming majority in the House of Representatives. This was long before the Dred Scott decision declared slaves to be property. A sad business all of it, but the Founders did the best they could for the time.