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.
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.
The meeting will be held on May 3rd.
This repair will take three days.
I went to school for the first time in three days.