Ok, thank you guys, following hafniumzinc guide I was able to set up my first MQTT device.
I must mention that my inital step was to try setting up broker and things through PaperUI -> No success!
After I switched to textual configuration it worked.
I hope UI setup will work in ver3 of OH…
I would also like to mention that setting up MQTT devices is more complex than with the old version (especially setting up a thing for every device). As far as I understand this is required to follow the logic behind OH things/channels logic.
Anyway, thanks again and have a nice day!