Albert Barnes on Revelation 9:18

By these three. Three things--explained immediately as referring to the fire, the smoke, and the brimstone.

Was the third part of men killed. Rev 8:7-12, on each of which verses we have notices of calamities that came upon the third part of the race, of the sea, of rivers, etc. We are not to suppose that this is to be taken literally, but the description is given as it appeared to John. Those immense numbers of horsemen would sweep over the world, and a full third part of the race of men would seem to fall before them.