Inventor 2018.3 is an update that lets you work with others who are on Inventor 2019.
AnyCAD for Inventor
Reference Future (newer) Versions of Inventor Part and Assembly Files
You can now reference an Inventor 2019 part or assembly file in Inventor 2018.
You must have Inventor 2018.3 installed to support opening 2019 part and assembly files. Inventor 2018.3 can be downloaded via the Autodesk desktop app or from https://manage.autodesk.com.
Use the Open, Place, or Derive command to create a reference to a future version of an Inventor part or assembly file.
Migration: If you open and save the file in a newer version of Inventor (or use Task Scheduler to migrate files with references to future versions) the future version references are migrated to standard Inventor Place and Derive relationships.
For more information, see About AnyCAD for Inventor.
AnyCAD Enhancements
Newer Versions of AnyCAD Formats are Supported
You must have the latest updates installed for your version of Inventor. Updates can be downloaded via the Autodesk desktop app or from https://manage.autodesk.com.
For more information, see About Importing Files from other CAD Systems.