<p>Three credit classes are more common at schools that run on a quarter system. At those schools, a 3 credit class meets 3x week for 50 minutes or 2x week for 75 minutes, but for only 9-10 weeks (which is how long a quarter lasts) as opposed to a semester which is 12-14 weeks.</p>
<p>Semester classes cover more material than do quarter classes. I’ve taken [grad] classes at schools which use both quarter and semesters unit systems and which used 3 and 4 credit units respectively. The amount of work for each is about the same DURING THE ACADEMIC UNIT PERIOD, but since the semester lasts longer a 4 credit class overall requires more work.</p>
<p>Many, many years ago when dinosaurs roamed the earth (and I was an undergrad fending off pterodactyls on the way to class), 3 credit courses were the norm for a 150 minutes per week semester long course. However, that changed about 15-20 years ago in order to help standardize course weighting since a number of major schools (esp on the West Coast) run on academic quarters instead of semesters.</p>