I recently encountered the claim (that I see often enough) that the U.S. Constitution defined slaves as 3/5 of a person. That claim is actually false. The Constitution did no such thing. What it did is count them as 3/5 toward representation, which was a compromise between those who didn't want them represented and those who thought they should count fully. Here is what the actual wording said:
Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons.
The wording actually assumes they are full persons. It distinguishes between the contribution to the census from free persons and the contribution from other persons. It's 3/5 of the number of other persons that gets added to the number of free persons. It's not that slaves are 3/5 of a person.
And for the record, it was those who opposed slavery who didn't want them counted and those who favored it who did, because counting them as full persons would mean more representation in Congress for their states (and yet the voting for those states wouldn't involve the slaves voting, of course, so it's even more influence for the slave-holders if they counted fully).
If we take the constitutional wording to imply that slaves were only viewed as 3/5 of a person, we should also conclude that abolitionists must not have thought slaves were real people, because they wanted them counted as zero, and slaveowners must have thought they were indeed real people, because they wanted them counted as full persons. It's not as if those who favored slavery were defining slaves as less than full persons. It was those who opposed slavery who didn't want their slaves counting toward representation when they didn't have representation who were behind this.
Interestingly, the roles had been reversed for the debate over an amendment on this for the Articles of Confederation, because that debate was over how much in taxes the states had to pay, where the non-slave states wanted slave states to pay more due to their higher population. You would have more success making that argument in this case, because at least the roles line up that way, but that would misunderstand what the issues were.
It had nothing to do with their actual view of the moral status or personhood status of slaves but was about how much political influence states would have, and the Articles of Confederation debate about the same exact issue had been about how much in taxes they would have to pay. Which issue it was about determined which stance each side took, and they completely reversed their positions when the issue changed to make the opposite view favor them. So there's simply no claiming that this was about defining the personhood of slaves or anything. It was simply about how to calculate populations for political results, and those who argued for each side compromised between counting them for certain purposes and not counting them for those purposes by proposing the 3/5 count.
There are plenty of things you might disagree with about how slaves were treated, and it is indeed unfair to be counted at all for representation but not being represented (but we do that with children still). Nevertheless, it's simply false that the Constitution defined them as 3/5 of a person, as if that judgment in particular reveals a view that slaves were viewed as not fully persons. It does no such thing, because it's not about that issue at all. To find evidence that people believed such a thing (and I'm not saying there is no such evidence), it doesn't do to cite what the Constitution says about this issue.