It seems the Processing options in QGIS 3.0 no longer include R Scripts (they are not listed as available along with SAGA, GRASS, and GDAL). Is this a change in this version of QGIS or have I had a problem with installation?
Actually it's not broken - they were removed from the master install, and are waiting for someone to step up and maintain them as a plugin. ... The situation won't change with 3.0.1/3.2 until someone steps up.
In QGIS 3, the R processing provider is not included in core, but
Processing R Provider plugin is now available (for QGIS 3.4 and later): https://plugins.qgis.org/plugins/processing_r/. You can enable it in QGIS 3 with the plugin manager (Plugins-> Manage and Install Plugins...).
The current version is 1.0.1. I had an experimental version installed, and 1.0.1 was not recognized as an update, so I had to uninstall the experimental version to be able to install 1.0.1.
Good news: Version 0.9.0 of the Resource Sharing plugin (http://qgis-contribution.github.io/QGIS-ResourceSharing/) was made available on http://plugins.qgis.org today, 15 February 2020 (https://plugins.qgis.org/plugins/qgis_resource_sharing), and it includes support for R script collections. See https://gis.stackexchange.com/a/341008/22646 for more details, and for information about available collections and repositories.