I have a gp tool I wrote in Pro that uses some python libraries that exists only for Python 3 and I would like to publish a GP service for that tool.
As far as I know (see here: FAQ: What version of Python is used in ArcGIS? ) Python 3 is not installed with Server.
Is there a way to do it?
There's no way to use modules/libraries for a version of Python that differs from the version shipped with ArcGIS Server, unfortunately, as GP services will use Python 2.7.
I think the difference is whether the OP wants to run GP services using python 3, (which I can't see being possible), vs running standalone scripts on the Server machine using python 3.