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: