There's been several posts to GIS StackExchange along the lines of:

Where's my .qgis3 folder?

Prior to QGIS 3, the .qgis/.qgis2 folder was found under your home directory. At version 3, the folder has moved to a more standard profile location for your operating system.

There are a couple of ways to determine where the folder is located:

  • Use the Settings->User Profiles->Open active profile folder menu item
  • Use QgsApplication.qgisSettingsDirPath from Python or the console

Here are the "standard" locations for Linux, Mac, and Windows, as found under your HOME directory:

  • Linux:
    • .local/share/QGIS/QGIS3/profiles/default
  • Mac OS X:
    • Library/Application Support/QGIS/QGIS3/profiles/default
  • Windows:
    • AppData\Roaming\QGIS\QGIS3\profiles\default

To get the location of your plugins directory, just add python/plugins to the appropriate location above. For example:

AppData\Roaming\QGIS\QGIS3\profiles\default\python\plugins

From the Settings->User Profiles menu, you'll notice a New profile item. This allows you to have multiple configurations of QGIS 3. Each new profile is created in the same "base" location as listed above. For example:

AppData\Roaming\QGIS\QGIS3\profiles\new_profile

4:31 PM under pyqgis , qgis , qgis3