Why did the Southern states want the Constitution to classify a slave as three-fifths of a person?
For power in the House of Representatives.
There were about 2.3 million slaves in southern states by 1860. Our constitution provides that the number of representatives has is in proportion to its population. The house had 237 members. Those 237 members had to represent equal number of people except in states with fewer than 130,000 residents which would get a single representative. Even so, the north had the largest portion of the white male population. By getting a 3 fifths count the south gained an addition 1.4 million residents which meant more representation in Congress.