A.T. Robertson Commentary Revelation 16:13

"And I saw [coming] out of the mouth of the dragon, and out of the mouth of the beast, and out of the mouth of the false prophet, three unclean spirits, as it were frogs:" — Revelation 16:13 (ASV)

Coming out of (εκ alone, no participle ερχομενα).

Of the dragon (του δρακοντος). That is Satan (12:3,9).

Of the beast (του θηριου). The first beast (13:1,12) and then just the beast (13:14ff.; 14:9,11; 15:2; 16:2,10), the brute force of the World-power represented by the Roman Empire" (Swete).

Of the false prophet (του ψευδοπροφητου). Cf. Matthew 7:15; Acts 13:6; 1 John 2:22; 4:3; 2 John 1:7. Identified with the second beast (13:11-14) in 19:20; 20:10. So the sixth bowl introduces the dragon and his two subalterns of chapters Re 12; 13 (the two beasts).

Three unclean spirits (πνευματα τρια ακαθαρτα). Out of the mouths of each of the three evil powers (the dragon and the two beasts) comes an evil spirit. See the use of mouth in 1:16 (9:17f.; 11:5; 12:15; 19:15,21) as a chief seat of influence. In 2 Thessalonians 2:8 we have "the breath of his mouth" (the other sense of πνευμα). For ακαθαρτον (unclean) with πνευμα see Mr 1:23f.; 3:11; 5:2ff.; Acts 5:16; 8:7. Christ expelled unclean spirits, but His enemies send them forth" (Swete). See Zec 13:2 "the false prophets and the unclean spirits."

As it were frogs (ως βατραχο). Cf. Exodus 8:5; Leviticus 11:10f.. Old word, here alone in N.T. Like loathsome frogs in form.