three unclean spirits
These are spirits of demons. See commentary on Revelation 16:14. See Three: Life, Resurrection, Completeness, the Trinity.
Their comparison with frogs alludes to their uncleanness, being aquatic animals lacking scales (Lev. Lev. 11:9-12; Deu. Deu. 14:9-10). They also recall the plague of frogs in Egypt (Ex. Ex. 8:2-13; Ps. Ps. 78:45; Ps. 105:30). See The Plagues of Egypt and the Tribulation.
They are the elect agents to awaken the world to the attempt to abolish God from the earth; and they are frog-like in that they come forth out of the pestiferous quagmires of the universe, do their work amid the worlds evening shadows, and creep, and croak, and defile, and fill the ears of the nations with their noisy demonstrations, till they set all the kings and armies of the whole earth in enthusiastic commotion for the final crushing out of the Lamb and all His powers.1coming out of the mouth
of the dragon . . . of the beast . . . of the false prophet
The three spirits correspond to the three personages of the antitrinity : the dragon, the Beast, and the False Prophet. All three work together with a unified goal of drawing the nations to battle.
The dragon is Satan and the devil , both names which indicate his slanderous accusations (Rev. Rev. 12:9+). When he speaks a lie, he speaks from his own resources, for he is a liar and the father of it (John John 8:44). The Beast is known for his blasphemous mouth (Dan. Dan. 7:8, Dan. 7:11, Dan. 7:20, Dan. 7:25; Dan. 11:36; Rev. Rev. 13:6+) and the False Prophet, although appearing like a lamb, speaks like a dragonHe tells those who dwell on the earth to make an image to the beast (Rev. Rev. 13:11+). Each of these would be highly influential on their own, but aided by unclean spirits, their deception is especially effective. See commentary on Revelation 13:14.
1 J. A. Seiss, The Apocalypse: Lectures on the Book of Revelation (Grand Rapids, MI: Zondervan Publishing House, 1966), 378.