Harvey Mudd students need to have a four course concentration within their eleven H/SS courses, which has to advance beyond introductory level courses. Two of the eleven courses must be writing intensive. The general pass/fail grade option rules (no more than one per semester, and one per department in an academic year, with all H/SS counting as one department) mean that no more than four H/SS courses may be taken pass/fail.
https://www.hmc.edu/hsa/curriculum/graduation-requirements/
MIT requires that three or four of the eight H/SS courses be a concentration; it appears that any allowed concentration includes an upper level course. Two communication intensive courses must be included. All H/SS must be taken for a letter grade except for frosh year P/D/F grading.
https://registrar.mit.edu/registration-academics/academic-requirements/hass-requirement
Caltech requires that 36 units of each of H and SS be divided between introductory and advanced courses (an additional 36 units of elective H/SS is also required to make 108 units). Two frosh-level humanities courses must be writing intensive. The advanced H/SS courses may not be taken pass/fail (but introductory level courses are not restricted, and all frosh year courses are pass/fail).
http://www.admissions.caltech.edu/documents/2367/Core_Curriculum_Requirements_2014.pdf
Really, all three have fairly heavy H/SS requirements compared to many other colleges. Compare to Brown (no H/SS requirements except writing, except that ABET accredited engineering majors need four H/SS courses, or about 13%).