Of course slaves weren’t allowed to vote, and 3/5ths meant that the South would get more representation as recognition of the economic strength they were bringing to the union.
It’s not like the Northern vs Southern states were arguing about whether a slave’s vote was worth 0, 3/5, or 1. This was about ascribing more (or less) power to slave owners.
The south wanted 1, the north wanted 0.
The 3/5 compromise was one of the forces that eventually brought about the end of slavery. It didn’t increase the south’s representation, it decreased it.
What I’m saying is that it “increased” the representation versus 0. Which is equivalent to your formulation that it “decreased” the representation from 1. In other words, 3/5 is more than zero and less than 1. No disagreement here.
My point is that it wasn’t about giving rights to black people, it was an argument about how much representation slave owners would have.