[DiogenesLamp]

If they wanted to end slavery so bad, why not just end slavery right then and and not put the Three-Fifths Compromise in the Constitution?

They needed the slave holding states to provide a stronger front against English efforts to retake the colonies. Without the slave holding states, they would have been conquered by the British. With the slave holding states added to their defensive capability, the British were discouraged from attempting to grab any of the states back.

Also bear in mind that in 1787, the vast majority of states were slave states.

[Svartalfiar]

Also bear in mind that in 1787, the vast majority of states were slave states.

In 1787, we had one free State, Massachusetts.

Pennsylvania, New Hampshire, Connecticut, and Rhode Island had all passed laws eliminating it gradually (mostly preventing new slaves, but grandfathering in existing ones), and they didn't fully get rid of slavery until the 1840s-50s. Vermont banned slavery in her Constitution in 1777, but wasn't a State until 1791.