A.T. Robertson Commentary Revelation 9:18

"By these three plagues was the third part of men killed, by the fire and the smoke and the brimstone, which proceeded out of their mouths." — Revelation 9:18 (ASV)

By these three plagues (απο των τριων πληγων τουτων). Our "plague" or stroke from πλησσω, as in Lu 10:30 and often in Rev. (9:20; 11:6; 15:1,6,8; 16:9; 18:4,8; 22:18). It is used in Ex 11:1ff. for the plagues in Egypt. The three plagues here are the fire, smoke, and brimstone which proceed from the mouths of the horses.

Was killed (απεκτανθησαν). First aorist passive indicative of αποκτεινω, to kill, third person plural, though το τριτον is neuter singular because a collective idea. See same form in verse 20.