In general, Penn will be the choice for those Hum/SocSci subjects that benefit the most from an urban mindset: the newer, industrial-focused areas of study such as Economics, Political Science and Sociology.
Dartmouth has pockets of strength - in International Relations in particular - but overall it lags behind the other two. It’s essentially a small liberal arts college that is best compared to Amherst, Williams and Bowdoin. While Dartmouth has a good graduate business school from which you might be able to take a course or two, Penn is the obvious choice for anyone focused on Economics.
Brown is generally stronger the further you get from the modern, industrial world: Classics, early American History, Anthropology. For Psychology as well, Brown is probably best of the three.
As to Literature, Philosophy and History, they’re all solid.
For Linguistics, Penn is clearly the best.
Nb. If your interests involve anything related to computational linguistics, Brown is perhaps the best choice and Dartmouth is also very good. They’re all strong in computer science, with Brown especially strong in applied math and graphics-related algorithms.
Hope that helps.
Note that these are very different schools. Penn is the Big City and a huge research complex. Dartmouth is a LAC. Brown is something of a hybrid of the two.
Ask yourself whether you want to be with a more urban, feet-on-the-ground, pre-professional crowd (Penn), somewhat more laid-back types who like to ski and drink before they head off to Wall Street or grad school (Dartmouth), or wealthy, happy liberals who fall in between the two (Brown).
LAC, Slack or Urban Attack. Your choice.