At this moment (first half of 2018) this is still very much work in progress. Both the XProc 3.0 standard and the accompanying book are still under development.
More information about XProc 3.0 can be found at http://spec.xproc.org/master/head/
Publishing is planned somewhere beginning 2020.
Erik Siegel
Xatapult Content Engineering
+31 6 53260792