There are a bunch of stuff that make this hard to calculate. One is that cases are undercounted, while deaths are almost certainly not. Another is that deaths are correlated to available healthcare. 3x cases would mean that the standard of care goes down, which means more deaths. More deaths would probably also mean increased "protective" measures. Risk tolerance (and risk of infection) is elastic. Seeing 3x more deaths would certainly mean at least some people would take it more seriously, which reduces risky behavior and slows down cases, etc.

So you're right, if worst case everyone in the US gets infected, we're in for a lot more deaths, but aside from an apocalyptical R=100 type disease, not everyone is getting infected. The more infections there are, the more preventative measures are taken, etc.

It's a hard problem.