go get attempts to resolve each package named in its arguments to a module. In the last command, since no module in the build list provides rsc.io/quote/v3, the go command will try to download the modules rsc.io, rsc.io/quote, and rsc.io/quote/v3 from the first proxy. The first proxy has rsc.io/quote, but that module doesn't contain the package rsc.io/quote/v3.
The go command should fall back to the next proxy in this case since it didn't find the package and didn't receive a terminal error from the proxy. Instead, modload.PackageNotInModuleError is treated as a terminal error, and the go command gives up without going to the next proxy.