VSTO with .NET Core 3.0

It seems like there is currently no alternative. There is a request to add this support in the future to .Net 5 https://developercommunity.visualstudio.com/idea/757925/please-port-visual-studio-tools-for-office-vsto-to.html

The recommendation currently seems to be using the Office JS platform to create add-ins. These will be compatible across platforms also (https://blogs.msdn.microsoft.com/deva/2018/04/02/office-developer-difference-between-office-web-addin-vs-comvsto-addin)