I pay ~100/month per car for full coverage on two fairly new cars and $500k in liability from the auto policy plus a $1m umbrella policy. And that's in CA which is comically expensive. I find it very believable that you can get excellent liability only coverage on 3 cars for $100/month depending on the state and drivers.

For comparison, I live in SF and am low-risk on all the dimensions you'd expect on HN, and I pay $100/month for non-owner coverage with similar limits - i.e., I don't own a car and my coverage only applies when I rent one. When I owned a car it was much higher, of course.