It does indeed mention 3 inventions by slaves, and then a number of inventions by free men. For some perspective, here's an accounting of the number of patents by year:
The whole point is that such evidence would not exist in the first place, so 3 examples are actually statistically significant and indicate that there are a whole lot more than that.
https://www.uspto.gov/web/offices/ac/ido/oeip/taf/h_counts.h...