three days a week
A set expression meaning 'three days per week'. Commonly used in schedules, habits, or frequency descriptions.
I go to the gym three days a week.
This class meets three days a week.
Means 'four days a week', following the same pattern.
Compound of 週 (しゅう, 'week') and 三日 (みっか, 'three days'). The reading みっか is the standard Japanese counting form for three days.