So the 3/5th Compromise wasn't found in Article 1, Section 2, Paragraph 3 of the United States Constitution?
I would suggest going over to the Library of Congress' website. You'll find the diaries of the founders during the first Constitutional convention. Most people don't even know this, but the 3/5ths clause wasn't to dehumanize slaves (and Native Americans), it was to take power away from slave owning states until they freed them.
Most of the founders were actually abolitionist, but in order to get the Republic, they had to have the slave owning states go along. The slave owning states wanted to have full representation based on their entire population, including slaves and Native Americans. This would give those states far greater representative power than the other states and they could vote slavery in perpetuity. The solution the abolitionists of the time came up with was the 3/5ths clause. They said outright that until they freed their slaves, they would not give them representative power based on the population count that included the slaves.
So they weren't dehumanizing slaves, they were taking power away from the slave holders.