Hi 12137022, welcome to the forum!

Is it possible to receive data from hmc5883l (i2c) sensors, through a raspberry pi 3.

This question is outside the scope of this forum. It probably is possible, I would expect there are guides out there for doing I2C on an RPi and possibly the specific sensor. And maybe someone here will know, too.

Treat this data on python and send it directly, without having to send it to a database?

What do you mean? Mango can have points that do not log, and you could just have a list in python that grows until you run out of memory. So, yes, that is possible, whichever you are seeking to avoid databases with.

I would like to monitor the sensors in real time and display in a graphical interface.

Once you've gotten your values into Python, you can queue them in a simple list and publish them to Mango all sorts of ways. This seems like it's duplicating the very recent question: https://forum.infiniteautomation.com/topic/3737/import-values-from-python-script