What kind of information are expected in the "Additional comments" section of a letter of recommendation?
Note: this is just my opinion, not necessarily the "correct" or "necessary" way to do this. I also agree with @aparente001 that you could highlight some other accomplishments of the student outside their academic work, or reflect on a particular struggle the student had and overcame if appropriate. Another alternative is an anecdote about the student that highlights one of the other characteristics you discussed or something else in their application.

That said, Sections 1 and 2 relate to the characteristics of the student, and you could write these sections without necessarily knowing anything about the university or program the student is applying to. Section 3 is an opportunity for you to say "this student would be a great fit for your university because _____." That reason might not be particularly about their academic performance or personal demeanor but rather something that ties the student to where they want to attend. If you don't know what the student's particular ambitions are, it would be completely appropriate to ask them.

Essentially you are corroborating the student's statement of purpose or similar essay they might be including in their application (you could ask them for this, as well). Along the lines of diversity, I'd be a bit wary of saying "because this student checks categories A, B, and C they'd be great for your school's statistics!" (I know you wouldn't really say that, but I'm just pointing out that the school will already have whatever diversity-related information they need to make those decisions) However, there might be related issues you can highlight. Maybe the student has aspired to be the first generation in their family to attend an institution of higher learning. Maybe they want to go to University X because of their excellent program in the agricultural sciences. Maybe their parents are both physicians and the student wants to go to University Z because their broad liberal arts education will allow them to seek what really drives them. It all depends on the student and their goals. Anecdotes are great too. This is your opportunity to talk about the time your student gave a talk in class on a particular topic, or produced a particular project, or took on a role in the school play, etc, and showed a particular passion or aptitude. 

I'd avoid leaving it blank if you want to submit a solid recommendation.