Sorry for late reply! I actually invited the University to translate correctly their name in the website. Roma Tre is not a proper name! It's simply the chronological order due to the date of establishment. Moreover, students and professors usually refer to the four public universities as Rome Uno (for La Sapienza), Roma Due (for Tor Vergata), Roma Tre and Roma Quattro (for Foro Italico). Despite if it's write or wrong, it's a matter of fact. For example many websites of both professors (e.g. Prof. Giancarlo Corsetti) and alumni (e.g. Andrea Polo, University of Oxford) cite the University as University of Rome III. I personally think this is the clearest and simpliest way to refer to the University. The Paris University System adopts this method.