The 3rd day of the month (date).
彼の誕生日は3日です。
His birthday is on the 3rd (of the month).
会議は来月の三日に予定されています。
The meeting is scheduled for the 3rd of next month.
三日は祝日なので銀行は休みです。
The 3rd is a holiday, so banks are closed.
A period of three days; for three days.
旅行は三日で終わる予定だ。
The trip is scheduled to end in three days.
この仕事は３日かかると言われた。
I was told this job would take three days.
風邪は三日ほどで治った。
The cold cleared up in about three days.