At the same time, don’t make the opposite mistake of assuming “everything your history teacher told you was a lie.” The New York Times made some bold assertions in the 1619 Project and has had to walk back some of the most fundamental assertions: https://www.nytimes.com/2020/10/09/opinion/nyt-1619-project-.... If you go through and strip out all the rhetoric based on factual premises that have been abandoned or debunked (even by the left, see: https://jacobinmag.com/2019/08/how-slavery-shaped-american-c...) what’s left is a really interesting take on aspects of history your AP US History curriculum doesn’t cover, but which doesn’t fundamentally contradict what you learned in school.
With respect to this particular issue: the AP US History curriculum correctly taught what the 3/5 compromise was (and it was what it was). And it also correctly taught that the Connecticut Compromise was about small states versus big states, not slavery. It was a slave-owning southern state, Virginia, that proposed proportional representation in both houses. And the small states that benefited from the structure of the Senate included both northern states and southern states.
The connection is also a weird thing to bring up today, because most of the small states that benefit from the structure of the Senate today were free states. Several, like north and South Dakota, were split up when admitted to break the power of the slave states in the Senate. Midwestern states like Iowa and Minnesota made a huge contribution to beating the south. Iowa sent troops to fight before the union army was even ready to go to war; they sent the largest percentage of their population to war of any state; and they had the highest percentage of casualties: https://valleynewstoday.com/news/local/iowa-played-a-large-r....
As it happens, on my fraternal grandmother's side we had family that fought in the Civil War for the Union from Iowa. One day I'll look all that up and figure out what units they fought in and where they're buried.
I'm not trying to say that "the EC was about slavery" is a correct narrative or even more valid than "it was about states". The point I'm trying to make is unrelated to the specific question about the EC and slavery. The point I'm trying to make is that there are hundreds of narratives for almost every historical event. Motivations that range from the smallest microhistory to the largest historical narratives. And that "APUS said it was about X" is going to miss a whole lot of interesting scholarship.
Again, I'm not trying to make any statement about the EC. I'm trying to make a broader suggestion about the limitations of high school history classes.
There were two initial proposals for Congress: the New Jersey Plan and the Virginia Plan. (Virginia, of course, being the leading slave-owning state.) Under both plans, the President would have been elected by Congress. There were concerns that having Congress elect the President would jeopardize separation of powers. So the Electoral College was created, with one elector for each member of Congress. It had nothing to do with protecting the interests of slave-holding states, but instead was designed to make the Presidency more independent of Congress.
Indirect election of the executive is somewhat anti-majoritarian, but also extremely common. Justin Trudeau was not elected Prime Minister of Canada by the general population. He was elected by about 80,000 eligible members of the Liberal Party, and became Prime Minister when the people elected other members of his party to the Parliament. I will add that the leading proponent of direct election of the President and the Senate was Andrew Jackson, strongly opposed abolition and was generally a horrible person.
Now, the New Jersey Plan would have allocated one member of Congress to each state. It was also supported by New York, Delaware, and Connecticut. Of those, only Delaware had a significant enslaved population. The Virginia Plan created a two-chamber legislature with proportional representation based on population. Yes, slave-holding Virginia was the one that wanted proportional representation. The Connecticut Compromise that led to the current allocation really was about small versus large states, not slavery: https://en.wikipedia.org/wiki/1790_United_States_Census. Large states like Massachusetts and Pennsylvania had few to no enslaved persons, and small states like Georgia and Delaware had significant numbers of enslaved persons.
The 3/5 compromise was designed to reduce the interest of slave-holding states. The number originated in an amendment proposed to the Articles of Confederation, which proposed to set the tax obligation for each state to the federal government "in proportion to the number of inhabitants of every age, sex, and quality, except Indians not paying taxes." https://en.wikipedia.org/wiki/Three-fifths_Compromise#Confed.... Virginia complained that this would count enslaved persons, thus increasing Virginia's tax obligations. Eventually, they settled on counting 3/5 the number of enslaved persons towards the state's tax obligation. That same compromise came up later in the apportionment context.