According to the RC release summary it was meant to be possible to remain on a PCL (as opposed to migrating to .net standard).
We’re now building with .NET Standard 2.0, using the magic of multi-targeting, while still supporting the PCL profiles and other .NET Standard versions you may need. Notice how much faster your projects load when you use .NET Standard project types! from blog.xamarin update-to-xamarin-forms-3-0-pre-release-available-today
I have tried this but attempting to upgrade to xf 3 yields the error:
Package Xamarin.Forms.Pages 3.1.0.697729 is not compatible with portable46-net451+win81 (.NETPortable,Version=v4.6,Profile=Profile44). Package Xamarin.Forms.Pages 3.1.0.697729 supports: netstandard2.0 (.NETStandard,Version=v2.0)