This verse is but an introduction to the other three angels sounding, declaring that the times which were to follow would be much more full of miseries and woes to the inhabitants of the earth; by which I understand all those countries which lately were subject to the Roman empire. Others understand the more earthy, unsound, hypocritical part of the church. The woe is thrice repeated, either to show the greatness of the calamities, or rather correspondently to the number of the angels yet to sound.
Related Commentaries of Revelation 8:13
Revelation 8:13
13 And I beheld, and heard an angel flying through the midst of heaven, saying with a loud voice, Woe, woe, woe, to the inhabiters of the earth by reason of the other voices of the trumpet of the three angels, which are yet to sound!