I\'ve been using Saxon XSLT and XQuery processor - Free home edition for some of my .NET
apps. I need only XSLT 2.0
(since .NET does not support XSLT 3
XmlPrime https://www.xmlprime.com/xmlprime/ is a pure .NET implementation of XQuery 3.1 and XSLT 2.0, it was developed for the .NET framework and the latest version 4 also to work with Mono.
I have installed the current trial version of 4.1 and have written a .NET Core 3 console application with VS 2019 where I have added the two assemblies XmlPrime.dll and XmlPrime.ExtensionMethods.dll of the XmlPrime installation directory as references to the .NET core project.
Two simple examples adapted from https://github.com/XmlPrime/Examples/tree/master/Query and https://github.com/XmlPrime/Examples/tree/master/Transform then run fine to execute XQuery 3 and XSLT 2 in the .NET Core 3 app.
So technically XmlPrime, at least with some superficial tests, seems an option for .NET Core 3, I don't know about the state of their project however, as far as I know they only occasionally update their trial version and have stopped to sell commercial licenses online. You might need to enquire at the contact details of https://www.xmlprime.com/xmlprime/contact.htm to ask whether they sell full licenses some other way.