Which amendment were slaves counted as only three fifths of a person?

The "Three-Fifths Compromise" was not a constitutional amendment, but rather part of the actual Constitution written in 1787 (Article 1, Section 2, Paragraph 3), as an attempt to get both slave states, who wanted slaves to be counted as people because of taxation reasons, and non-slave states, who didn't want slaves to be counted as people, to ratify the document.