noun
third day of the month; the 3rd
Calendar-date use. With a month name, 三日 is read みっか, as in 五月三日.
会議は五月三日にあります。
The meeting is on May 3rd.
締め切りは来月の三日です。
The deadline is the third of next month.
noun
Duration use. 三日 can stand alone as a time span, and 三日間 is also common when emphasizing the full length of three days.
三日待ってください。
Please wait three days.
彼は三日間ほとんど寝ていない。
He has hardly slept for three days.
Means either the 2nd day of the month or two days; it follows the same date-or-duration pattern as 三日.
Means either the 4th day of the month or four days; note the special reading よっか.
Refers only to a duration of three days, not the third day of the month; 間 makes the time-span meaning explicit.
Native Japanese calendar and counter form read みっか. The kanji 三日 literally represent 'three' plus 'day'; the same spelling is used for both a calendar date and a three-day duration.