A.T. Robertson Commentary Luke 3:7

"He said therefore to the multitudes that went out to be baptized of him, Ye offspring of vipers, who warned you to flee from the wrath to come?" — Luke 3:7 (ASV)

To the multitude that went out (τοις εξπορευομενοις οχλοις). Plural,

Multitudes . The present participle also notes the repetition of the crowds as does ελεγεν (imperfect), he used to say. Matthew 3:7–10 singles out the message of John to the Pharisees and Sadducees, which see for discussion of details. Luke gives a summary of his preaching to the crowds with special replies to these inquiries: the multitudes, 10,11, the publicans 12,13, the soldiers 14.

To be baptized of him (βαπτισθηνα υπ' αυτου). This is the purpose of their coming. Matthew 3:7 has simply "to his baptism." John's metaphors are from the wilderness (vipers, fruits, axe, slave boy loosing sandals, fire, fan, thrashing-floor, garner, chaff, stones).

Who warned you? (τις επεδειξεν υμιν; ). The verb is like our "suggest" by proof to eye, ear, or brain (Luke 6:47; Luke 12:5; Acts 9:16; Acts 20:35; Matthew 3:7). Nowhere else in the N.T. though common ancient word (υποδεικνυμ, show under, point out, give a tip or private hint).