How to prevent users from turning off GPS, Wifi and Bluetooth?

You cannot prevent them from turning GPS, WIFI and Bluetooth off. What you can do is have an implementation as below or use this library.

https://github.com/KI-labs/gps-permission-checks-livedata

You can't, obviously for security reasons. If you want to achive something like that you'll probably need to modify the devices ROM. You should create a BroadcastReceiver and keep tracking Internet and Bluetooth connection changes, than you can properly handle it when user disconnect them pausing the service, showing a dialog, finishing the application or whatever you need to do.
It would be pretty weird if an app could have some control of user settings, imagine if you install an app, then suddently you can't disable wi-fi anymore until you unistall it. You can't do that for a good reason