What field were you supervising?
3 years in CS is, imo, a little tight but not bad. But I always wondered how people in other fields (stem and otherwise) did it. CS simply doesn't have to deal with the extraneous factors that come with, e.g., biology or climatology.
One of the factors making the finish-in-three-years target very difficult to meet is that the thesis committees would include faculty from different fields and backgrounds, often with differing expectations for what a Ph.D. thesis should include. It was not unusual for a committee to ask a student to make a variety of substantial changes and additions to a thesis draft before submitting. That revision process could take six months or longer; because of the schedule for graduate coursework and defenses, that alone would push the student beyond the three-year target.
Add in all of the other stumbling blocks that might come up in graduate school—family issues, financial problems, an advisor who retires or gets ill, etc.—and three years was no longer a realistic goal in that department.