Re: ArcGIS Diagrammer 10.3 indicates that there will not be a 10.3 of ArcGIS Diagrammer, because Diagrammer uses a third party charting library that is no longer available. They suggest to stay at 10.2, but for many people that won't be an option.
There is X-Ray for ArcCatalog, but is only available for versions up to 10.2 and I have no idea if there is a 10.3 version coming soon: http://www.arcgis.com/home/item.html?id=9ea218ff575f4a5195e01a2cae03a0ae
There are a number of options (not specifically for 10.3) discussed in this thread:
Where can I find the various Esri tools to design a geodatabase? - Geographic Information Systems St...
The alternative would be using Python, but that may include some serious programming...