Looking at the tapestry of the World Hymn, the transitional point between the Second and Third Age seems to be the earthquakes that began the Ages of Ruin. The transitional point between the Third and Fourth Age seems to be the Great Dying, or possible a large invasion of Orcs and Goblins.
These are notable events taking place in the Third Age, listed (approximately) chronologically:
- The world is wrecked by earthquakes.
- The Beast Herds appear.
- The Highborn Elves of Vetia came under pressure from barbarians (Beast Herds and Orcs) and Dwarves . They pleaded for help from Celeda Ablan but non came. 
- There seems to be some sort of infighting among the elves.
- The events of the Great Dying takes place in Naptesh. 
- Naptesh falls. 
Unless otherwise specified, text that green italic in the list above uses the World Hymn poem and tapestry as its source.
The World Hymn
Main article: World Hymn
The World Hymn is a dwarven carving of a poem that details all the nine ages of the world. It was later copied to an Equitan tapestry and supplemented with imagery. A transcript of the poem and the image for the third age are both shown below. 
The Age of Death, the loss of gold
A time when earth would swallow whole
Nor fae nor men nor orc nor we
All lost and toss’d and bent and broke
A time of pain to each ring Hold
The beasts did rise and burn and loot
No wall of shield or hold could stand
And fickle elves did leave and run
And men so quick to alter word
Do promise cut and render mute
The dead did rise on southern shore
An empire dead its heart torn out
No aid to come from human hands
The hour grew dark, and foes in count
So press’d were we, like ne’er before
We call on elves to help us hold
‘Gainst all that stood to break us then
And end the light of dwarven works
They turn’d their backs on us those days
Betraying us, with scandal bold
Now allies old do stand as foes
As mountains fall so too we break
And thus bring close this Age of Woes
- The World Hymn: the 3rd Age (T9A:FB Rulebook, v1.3.5, p127)
- Account by Rodomonte (The 9th Scroll, issue 9, p21)
- Informative text about the Dread Elves (The 9th Scroll, issue 6, p26)
- Naptaan Book of the Dead, translated by Abd al-Latif (T9A:FB Undying Dynasties, p11-12)
- Interview with Sigmund Selig, the Great Sage (The 9th Scroll issue #10)