In my experience going from two to three was a bigger change overall than one to two. Primarily because the kids now outnumber the adults. Also, particularly while they're little, you now have more kids than one parent has hands. So mom/dad can't physically restrain/direct all the kids by themselves. One to two felt to me like roughly a doubling of work. And that was when we felt like we weren't sure how much more we could handle. Then with our third it felt like the exponential increase you mention and we laughed at our previous feelings. The same strategies and patterns will scale from one to two but not necessarily to three because you get outnumbered. So three felt like more of a sea change.

At the same time, the mental adjustment was easier. None to one was the biggest mental adjustment for me (wow, I'm a parent now! Oh God help I'm responsible for keeping another human alive). One to two and two to three were more like changes in degree rather than a complete paradigm-shift mentally. For number two and three I've already worked through the reality that I'm now a father.

Age difference has to play some role as well. Our three are very close in age. Maybe the experience is different when there is more distance between them.