Showing two methods to locate active config file.

Method 1)

Start Robomongo, select Logs button on bottom-left corner of program's main window

5:00:38 PM	Info: Settings saved to: C:/Users/some_user/.3T/robo-3t/1.1.1/robo3t.json

Method 2)

Robomongo config file locations:

Windows
    1.1 and later
    C:\Users\<user>\.3T\robo-3t\1.1.1\robo3t.json

    1.0
    C:\Users\<user>\.3T\robomongo\1.0.0\robomongo.json

    1.0-RC1
    C:\Users\<user>\.config\robomongo\1.0\robomongo.json

    0.9.x
    C:\Users\<user>\.config\robomongo\0.9\robomongo.json

    0.8.x
    C:\Users\<user>\.config\robomongo\robomongo.json   

macOS
    1.1 and later
    /Users/<user>/.3T/robo-3t/1.1.1/robo3t.json

    1.0
    /Users/<user>/.3T/robomongo/1.0.0/robomongo.json

    1.0-RC1
    /Users/<user>/.config/robomongo/1.0/robomongo.json

    0.9.x
    /Users/<user>/.config/robomongo/0.9/robomongo.json

    0.8.x
    /Users/<user>/.config/robomongo/robomongo.json     

Linux
    1.1 and later
    /home/<user>/.3T/robo-3t/1.1.1/robo3t.json

    1.0
    /home/<user>/.3T/robomongo/1.0.0/robomongo.json

    1.0-RC1
    /home/<user>/.config/robomongo/1.0/robomongo.json

    0.9.x
    /home/<user>/.config/robomongo/0.9/robomongo.json

    0.8.x
    /home/<user>/.config/robomongo/robomongo.json