What is the difference between Revit 2018 and Revit 2018.3 versions?

Answer 1

That is a long list to answer but here links to help you see what it was when 2018 was first released withw it became the 2018.3 update.

2018 release

https://blogs.autodesk.com/revit/2017/04/13/introducing-revit-2018/

in the "2018.3" update

https://knowledge.autodesk.com/support/revit-products/learn-explore/caas/sfdcarticles/sfdcarticles/Released-Revit-2018-Updates-Add-ins.html?st=Revit%202018.3

This is another link to get more of the feature information too

https://blogs.autodesk.com/revit/category/architecture/