Why does Hebrews 3:1 call Jesus an apostle?
The original Greek word in this verse is apostolos (Ἀπόστολον Strong's G652). Strong's Concordance defines it as follows:
Definition: a messenger, one sent on a mission, an apostle
Usage: a messenger, envoy, delegate, one commissioned by another to represent him in some way, especially a man sent out by Jesus Christ Himself to preach the Gospel; an apostle.
Note what Thayer's Greek Lexicon mentions in reference to this verse:
ἀπόστολον ... τῆς ὁμολογίας ἡμῶν the apostle whom we confess, of Christ, God's chief messenger, who has brought the κλῆσις ἀπουρανιος, as compared with Moses, whom the Jews confess, Hebrews 3:1 INT.
The topic "Apostle" in the Insight on the Scriptures corroborates this information:
The Greek word a·poʹsto·los is derived from the common verb a·po·stelʹlo, meaning simply “send forth (or off).” (Mt 10:5; Mr 11:3) Its basic sense is clearly illustrated in Jesus’ statement: “A slave is not greater than his master, nor is one that is sent forth [a·poʹsto·los] greater than the one that sent him.” (Joh 13:16) In this sense the word also applies to Christ Jesus as “the apostle and high priest whom we confess.” (Heb 3:1; compare Mt 10:40; 15:24; Lu 4:18, 43; 9:48; 10:16; Joh 3:17; 5:36, 38; 6:29, 57; 7:29; 8:42; 10:36; 11:42; 17:3, 8, 18, 21-25; 20:21.) Jesus was sent forth by God as his appointed and commissioned representative.
So while we usually understand this word applied to Jesus' 12 chosen disciples, it does also apply to Jesus himself.
[Unless otherwise noted, all scripture quotations from the New World Translation of the Holy Scriptures (Study Edition)]