The offices of Christ are his being prophet, priest and king. Technically we should speak of his threefold office.
It was described by Eusebius and more fully developed by Calvin.
The doctrine states that Jesus Christ performed three functions ("offices") in his earthly ministry – those of prophet (Deuteronomy 18:14-22), priest (Psalm 110:1-4), and king (Psalm 2)
In the Old Testament, people were set apart to these offices by means of anointing with oil. The term messiah means "anointed one" and so is readily associated with the threefold office. While the office of king is that most frequently connected with the Messiah, the role of Jesus as priest, which involves intercession before God, is also prominent in the New Testament, being most fully explained in Hebrews 7-10.
In the Westminster Shorter Catechism we have these questions and answers (see 23-26)
Q: What offices doth Christ execute as our Redeemer?
A: Christ, as our Redeemer, executeth the offices of a prophet, of a priest, and of a king, both in his estate of humiliation and exaltation
Q: How doth Christ execute the office of a prophet?
A: Christ executeth the office of a prophet, in revealing to us, by his word and Spirit, the will of God for our salvation
Q: How doth Christ execute the office of a priest?
A: Christ executeth the office of a priest, in his once offering up of himself a sacrifice to satisfy divine justice and reconcile us to God; and in making continual intercession for us.
Q: How doth Christ execute the office of a king?
A: Christ executeth the office of a king, in subduing us to himself, in ruling and defending us and in restraining and conquering all his and our enemies.