This thread has now been closed to direct all further updates about this release onto the latest announcement:
Please see https://releases.xamarin.com/beta-release-15-3-preview-3/ on the Release Blog for the details about this release, including the steps to update or downgrade as needed.
Please file a quick bug report using the "15.4 Preview" new bug form if you see any suspicious behavior in these versions that you wish to report.
Comment in this forum thread if your question is not related to a suspicious behavior after download, but instead one of the following:
EDIT September 26: Update link to Release Blog post for new versions.