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
three days; a period of three days
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.