3G for Raspberry PI3

Most of the 3G dongles work with Linux. They usually emulate a com port, and you are supposed to use "ppp" over that comport.
The "modemmanager" program can handle that.

Also, many emulate a "small cdrom drive". There they store the drivers for the module for windows. You often need to switch them using a special command into "com port" mode. Of course the windows driver knows how to do that. Under Linux the exact commands need to be figured out by someone. Many modes are already supported. If you are afraid dubblecheck the supported modems with modemmanager.

Then, once  your pi is up-and-running, the mobile provider will probably have just a few IP addresses for thousands of clients. This means that your pi will be assigned a private IP address: You cannot connect to that IP address from outside. So if it is not enough for your pi to simply report to "the internet" what it has measured, then you will have to set up a tunnel. I don't know if there are "services" for this. I have set up a tunnel from my pi-in-the-field to a server I already had. But hiring a cheap VPS does not cost more than about $5 or $10 a month.

Most of the 3G dongles work with Linux. They usually emulate a com port, and you are supposed to use "ppp" over that comport.
The "modemmanager" program can handle that.

Also, many emulate a "small cdrom drive". There they store the drivers for the module for windows. You often need to switch them using a special command into "com port" mode. Of course the windows driver knows how to do that. Under Linux the exact commands need to be figured out by someone. Many modes are already supported. If you are afraid dubblecheck the supported modems with modemmanager.

Then, once  your pi is up-and-running, the mobile provider will probably have just a few IP addresses for thousands of clients. This means that your pi will be assigned a private IP address: You cannot connect to that IP address from outside. So if it is not enough for your pi to simply report to "the internet" what it has measured, then you will have to set up a tunnel. I don't know if there are "services" for this. I have set up a tunnel from my pi-in-the-field to a server I already had. But hiring a cheap VPS does not cost more than about $5 or $10 a month.

I think that you should focus you attention on the Huawei USB dongle. It is a model hat has been adopted by many different carrier (Vodafone, G3, TIM and Telstra for sure, probably some USA carriers too). There are many models and due its large diffusion and adoption it is also available as non-dedicated module. It works with Ubuntu and many Debian platforms and I don't see reasons that it should not work on the Raspbian too. Below some links that can be helpful for you:

Raspberry Pi as a 3g (Huawei E303) wireless (Edimax EW-7811Un) router

https://www.raspberrypi.org/forums/viewtopic.php?f=46&t=38392

raspbian - Sending and receiving SMS via a huawei 3g Dongle E220 - Raspberry Pi Stack Exchange

https://shkspr.mobi/blog/2012/07/3g-internet-on-raspberry-pi-success/

Instead if you need a more sophisticated project totally under control of your hardware you can buy just a 3G data module that with very few wiring and soldering can be connected directly to the Raspberry PI board via the serial UART and has a wide set of AT comands to control it. The SIM800 module can be found on the market for less than 15$ and I am using it for my project Hugo based on the PSoC4 micro controller.

https://techship.se/products/simcom-sim800c-gsm-smt/

For this module exists many breakout boards for prototyping. The image below shows my prototype version of the Hugo project

Enrico