This thread has now been closed to direct further updates about this release onto the latest announcement:
See the MSDN blog announcement for highlights about new features.
See the Xamarin blog announcement for additional highlights about new Visual Studio Tools for Xamarin features on Windows in this release.
See https://releases.xamarin.com/15-6-preview-3/ for other details about the release, including the steps to update and links to the individual product release notes.
This release is now available as part of Visual Studio 2017 version 15.6 Preview and in the Beta and Alpha updater channels for Visual Studio 2017 for Mac and Visual Studio 2015 Tools for Xamarin.
Check the product release notes to see all the fixes and known issues.
Please report a bug for any additional new issues or errors in the release notes.
Comment in this forum thread if you get any errors when submitting an issue. Thanks!
EDIT January 25: Update link to Release Blog post for new versions.