Looks like it’s 380 in the Swiss Plateau (you might be mixing up sq km and sq mi), which puts its at about ~70% of the population density of the Netherlands as a whole.
But Belgium does suck. I drove from Amsterdam to Paris in the early 2000s, and Belgium stuck out as being obviously worse (dirtier) than the other two.
If you account for that, the effective density of Switzerland on the usable area is 600–700 people/km².