Working should be measured by the quality of work being done rather than the amount of time being spent on that work. It is the twenty-first century. It does not make sense that people are still working the same amount of time, if not more, as people fifty years ago. There are still professsions that need to work 40 hours a week for society to function, but not every professional needs to be working 5 days a week to accomplish the same amount of work.
A 3-day work week could be a practical reality for many professions. Many workers already work short or changing shifts or are faced with reduced work hours. Consolidating shifts to 3-days and making those shifts longer would benefit both an employer and employee. As an example, nurses and police officers frequently will work three twelve hour shifts in a week. This type of arrangement could work for many other professions.
For the vast majority of jobs working a 3-day work week is not only plausible, but would enhance productivity. Most worker would not mind work three 12 hours day in exchange for having 4-day weekends every week. There would be less time spent planning events on the employer's dime. Also less sick time since there wouldn't be the need to call in sick as often.
I would say yes. There are some jobs that even have 24 hour work shifts. With a very long work shift a 3-day work week is totally do-able. Even with a shorter shift than 24 hours, depending on the job and your cost of living you could find something with a 3-day work week. I'm sure it's hard to find one though.
A three day work week would be possible if it were made universal, and wealth distribution allowed it to be feasible. Even with those structures in place, however, there would always be some who would want to get away with working more in order to gain a material advantage over others. As soon as one person violates the rule and works 4+ days a week in order to buy a nicer house than his/her neighbours, then everyone else would want the same. Thus, regrettably, a 3 day work week is impossible due to basic human nature.