What is the significance of one-third in Revelation? Why is everything 1/3 in Revelation?
It comes up with the four angels, and the dragon:

8:7 The first [angel's trumpet] sounded, ... and a third of the earth was burned up, and a third of the trees were burned up, and all the green grass was burned up. 8 The second angel sounded, ... and a third of the sea became blood, 9 and a third of the creatures which were in the sea and had life, died; and a third of the ships were destroyed. 10 The third angel sounded, and a great star fell from heaven, burning like a torch, and it fell on a third of the rivers and on the springs of waters. 11 The name of the star is called Wormwood; and a third of the waters became wormwood, and many men died from the waters, because they were made bitter. 12 The fourth angel sounded, and a third of the sun and a third of the moon and a third of the stars were struck, so that a third of them would be darkened and the day would not shine for a third of it, and the night in the same way.
9:15 And the four angels, who had been prepared for the hour and day and month and year, were released, so that they would kill a third of mankind. 18 A third of mankind was killed by these three plagues, by the fire and the smoke and the brimstone which proceeded out of their mouths.
12:4 And [the dragon's] tail swept away a third of the stars of heaven and threw them to the earth.
As with any part of Revelation there are primarily four main theories.  Coincidentally most of them take the 1/3 literally regardless of the view as represented a significant judgment but minority (only 1/3) implicated result on the things in this world that otherwise sustain life. Here are the main views very briefly and overly simplified as pertaining one example:


  The first angel sounded his trumpet, and there came hail and fire mixed with blood, and it was hurled down on the earth. A third of the earth was burned up, a third of the trees were burned up, and all the green grass was burned up. (NIV Revelation 8:7)


Historicist

When barbarians attacked Rome in ~AD 410, there was burning or destroying everything in their path. Approximately 1/3 of the Roman Empire was ravaged.

Preterist

Similar to historicist but everything sandwiched around the sacking of Israel by Rome. Therefore destruction of actual vegetation in the land around Jerusalem. Let's assume around 1/3.

Futurist

The most literal of all views with everything in the future, probably a literal 1/3 of the earth destroyed by nuclear bombs or something like it in whatever kind of bombs that might exists in the unknown future.

Symbolic/Spiritual

Basically the obvious symbolism that can be imagined. For example 'whatever' kind of destruction which at any time damages the earth such as what we see on the news.  Maybe 1/3 is actually greater then what is actual but symbolism does not need to literally match the figure employed.

Note: I am a rare bird these days as I have unexpectedly switched from Symbolic to the Historicist view, which is really not that popular anymore. I started out in my teens as a futurist, moved to Symbolic, almost became a Preterist just before resting in my current view, Historicist. This is how I have became familiar with them over the last 25 years.