"America, Britain, and France negotiate a peace with Germany in 1939. Germany wins the war in Europe and stops at Poland and Ukraine, establishing Greater Germany."
Greater Germany was already established BEFORE the war broke out. If the western powers had negotiated quickly then Poland would have been restored to pre-war borders, Danzig comes to the Reich and a referendum in the Polish corridor would have been held. Totally reasonable in my view.
"America doesn’t lose a single soldier, but 20 million people, including 9 million Jews, die, concentration camps dot Europe, Britain is run by Oswald Mosley’s kin, and the Slavs are enslaved.
Check your time reference, the real war started only after Barbarossa in June 1941. Before that there were barely any damages and dead people. Britain would never have been in danger to be overrun had they negotiated.
"Did America, Britain, and France do the right thing, in your opinions?"
Definitely not. The results of the war were: 50+ million dead, millions of refugees, huge damages and physical losses, half of Europe under Stalin's boot.