OP, you shouldn’t give too much weight to an internet poster’s friend’s brother’s third hand account of any school’s environment. In every school, you’ll find social butterflies, party pals, balanced bros, bookworms, athletes and homebodies. You’ll gravitate towards your tribe but if party and sports are a huge factor then USC & UM should be a better environment than JHU.
As far as diversity goes. It depends upon what kind of diversity are you looking for. Here are racial demographics for all 3. Another post about JHU being Asian is wrong, it’s most diverse among three.
Whites Asians Hispanics Blacks International
JHU 37 24 13 6 11
USC 40 21 14 4 14
UM 61 14 5 4 7
Academics and research should be strongest at JHU but other two are no slackers. Prestige wise JHU is usually taken more seriously.
If money isn’t a big issue, go to JHU. If it is then go to USC. All good schools, you can’t go wrong.