I installed QGIS 3 with GRASS 7 this morning via OSGeo4W64, and the application opens up without any issues, however the GdalTools plugin is coming up as "Invalid", showing the error "This plugin is broken: Missing metadata file."
I can't uninstall/reinstall the plugin within QGIS, as it advises it is a core plugin. I can't find it in the repository either, which was my Plan B. I've uninstalled and reinstalled QGIS 3, along with all of the GDAL-related packages listed in OSGeo4W64, but nothing has corrected this error.
I have QGIS 2.18.17 installed on the same machine, which is working fine, however I've also noticed that I'm getting the same error for the fTools plugin in 2.18.17. Could be related but I can't be sure when that error first appeared – it may have been there for some time without me noticing.
Best Answer
I've resolved this by learning from Michael Stuyts' experiences in his comment above.
After upgrading from QGIS 2.18 to 3.0, I also had an empty folder "C:\OSGeo4W64\apps\qgis\python\plugins\GdalTools\tools". I removed the GdalTools folder and this has made the error disappear from the "Manage and Install Plugins" window.