The TLDR here is that this gave the South more voting power because they could partially count the non-voting slaves in coming up with the number of electors. And since the slaves couldn’t vote, the elector system allowed their masters to essentially steal their 3/5s of a vote.
It really is worth remembering that the Electoral College was designed the way it is to support slavery: https://www.theatlantic.com/ideas/archive/2019/11/electoral-college-racist-origins/601918/
The TLDR here is that this gave the South more voting power because they could partially count the non-voting slaves in coming up with the number of electors. And since the slaves couldn’t vote, the elector system allowed their masters to essentially steal their 3/5s of a vote.