The founders did not include “slaves” in the Constitution as such. Rather they referred to “free Persons” “Indians,” and “all other Persons.” I am not arguing that everyone thought of blacks (or Indians or any other non-white group) as of the same civilized status as whites. Nor am I arguing that slaves were not considered property in legal contexts. But regardless of any personal opinion, they are officially, legally, deliberately declared to be full Persons (not three-fifths) from the very beginning of the US Constitution, and every state which ratified or was admitted had to agree.
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. (from Article 1, Section 2)