Hi Michael Gayheart, thank you for your question relating to ArcGIS Diagrammer. Unfortunately the current version for 10.2 will be final release of Diagrammer. The reason for this is because Diagrammer uses a third party charting library that is no longer available. A 10.3 release of Diagrammer would require significant reengineering to support a new charting library.
If you require the capabilities of Diagrammer for future work then you may want to consider maintaining a computer with ArcGIS Desktop 10.2. Also, if certain capabilities are not available in 10.3 then I would suggest submit these features to the ArcGIS Ideas site for consideration in future release of ArcGIS.
Hope this helps.