I think that 3x is a pretty standard recommentation. For instace, a quick google gave a first link (from cliffnotes.com) that says this.

Here's a guideline to what freshman can expect throughout their college years: Each unit of credit requires 2 to 3 hours of outside classwork per week. So, a student carrying 15 credit hours should plan to spend 30 to 45 hours each week studying beyond the classroom.

per week, but not per day.

If a student has 3-4 classes a day, they can't reasonably spend 9-12 hours working solely on digesting those lectures that day. They'll have homework at the very least, likely some ongoing projects, normal human activities -- not to mention that in the US they probably also have a part-time (or full time) job.

> per week, but not per day.

Maybe we are talking about the same thing but maybe not. Most recommendations that I see (including the official one from the college where I work) say 2-3 hours outside class for every hour inside class. Maybe not the same day, but if you have a week with 3 hourly classes then the recommendation calls for 6-9 additional hours spent that week digesting the material of those class hours.

Sure, but you always need roughly that amount, or more, to get a deep understanding of new material. MIT operates on a 1:3 ratio of lecture to review, Cambridge is even higher.

Who takes 4 classes per day in college? A full load when I was in school was 12-15 hours of lecture time per week.

A full load is 12 credit hours per week (at the schools I attended in the US), but there were plenty of people who took more than that. At some schools the tuition caps out at a certain amount of credit hours (12 where I went) and you can take courses beyond that amount and it doesn't cost extra (in terms of money at least).