Well, I think the benefits of the 6-hour day would disappear then. IIRC correctly research shows that productivity falls steeply after 5 hours at work, with hour 6-8 being at 75% producitity, and after that 50% (approximately). So having 3x10 days would kill the benefit of increased producitivity during the time you are at work, while still costing as much.

As a policy, this sort of thing probably would not work but the statistic does not apply to all people. I'd be interested to see a large company which allowed the workers to decide freely. Measuring results would obviously be difficult.