See Announcement, Download, Release Notes
The new Xojo 2017r3 release brings us a 64-bit IDE. That is great news, but may require some updates on your plugins. The recommended version for MBS Xojo Plugins is the current version 17.5 which contains important fixes to load plugins at all on Windows. Over the year we fixed a couple of 64-bit related bugs, so please do not use the older ones and find those bugs. All plugin versions older than 16.0 do not load at all in the new IDE due to missing 64-bit code.
Due to limits on Windows for the number of plugins, you may not see all plugins loading. Some older plugins which are not supporting 64-bit well enough, may not load. When moving to 64-bit, please also consider moving code from QuickTime to AVFoundation on Mac.
Still you can build 32-bit applications, so no worry on that.
See also: Xojo and Real Studio versions supported by MBS Plugin, MBS Xojo Plugin 17.5 Announcement and
Windows DLL Limits and Xojo 64-bit IDE
PS: Blog post from Bob Keeney: Xojo 2017 R3