I uninstalled Qgis 2.18.16 osgeo4w deleting all related folders. When I installed the qgis 3.0.0 osgeo4w there were missing folders. For example one of the crucial folders missing was .qgis2 or maybe it's supposed to be .qgis3. I tried reinstalling again several times, the same results occurred. Does somebody know how to fix the installation?
There is no .qgis3 directory. QGIS now uses a more standard location depending on your operating system. You can determine where this is by opening the Python console in QGIS and using:
QGIS 3 does not come with a single settings folder because of a new functionality called user profiles. This allows to have different user profiles which all have their own settings folder and therefore each one with its own set of configurations, plugins, etc.
To find the current profile path folder (the equivalent of the former .qgis2 folder), use the following menu
Settings > User Profiles > Open active profile folder