Sixty-one companies in the United Kingdom joined a pilot program in June 2022 in which they reduced their employees' workweek to four days—with no reduction in salary—and eight months later, 91% of them say they have no plans to go back to a five-day week.
Makes sense the average producive work hours in an 8-hour day is like 4-5 from memory. The other hours are wasted. I suppose for stuff like essential work there would be argument that someone needs to be available over opening hours, but that'd be solvable with alternating shifts and hiring to fill the remainder.
In terms of three-day weeks, as a bonus, if you alternated the day off between a Monday and Friday week-on-week, you'd get fortnightly four-day weekends.