- To speak no word that is not true
- To make no weapon with which one man may kill another
- Never to use the One Power as a weapon except against Darkfriends or Shadowspawn, or in the last extreme defense of her life, the life of her Warder, or another Aes Sedai
These oaths were never necessary in the past, however sometime between the Trolloc Wars and the War of the Hundred Years it was decided that Aes Sedai needed to show the world that their use of the One Power was not a threat, and so the Three Oaths became part of the Aes Sedai raising ceremony.
In TWoTC it is stated that all three oaths were in place by the Trolloc Wars, certainly by the end.
Phrasing sentences so as to lead the listener to conclusions the speaker desires without saying anything false, are "breath and bread" to Aes Sedai. They cannot lie outright though, unless they are Black Ajah.
For an Aes Sedai to use the One Power as a weapon it is necessary that her or her Warder, or another Sister, are in a situation of danger, so they can also circumvent the third oath creating situations of danger as described above.
Aes Sedai lifespan
Recently it was discovered that use of the Oath Rod was actually reducing the Aes Sedai lifespans by half. When it was decided to use the Oath Rod for the first time, if they did so knowing that it would reduce significantly their lifespan, they had to have strong motivations.
Most probably the main motivation was the emerging for the first time since the Breaking of channelers aligned to the Shadow (the Dreadlords) and maybe the first suspicion of the presence of a Black Ajah (which eventually found a way to circumvent the oaths, replacing them with other dark ones).
Later women raised to the shawl were not told about the shortening of their lifespan, and so knowledge of the effect was lost.
The Ageless Look
In the days after the Time of Madness Aes Sedai expected to live as much as seven hundred to eight hundred years, barring accidents. With the use of the Oath Rod their lifespan was reduced to a maximum of three hundred and fifty years, and the phenomenon known as Aes Sedai "agelessness" came about, which meant that it was simply impossible to put an age to the Aes Sedai. The cumulative effect of the Three Oaths is what produced this agelessness. There was not agelessness during the War of the Shadow, of course, and little or none during the Compact of the Ten Nations.
Though the actual making of Power Wrought weapons is forbidden, it is curious to note that after the rediscovery of Cuendillar, the weave is not used on the weapons and armor of the forces of the Light against the Shadow. Since the weapon is already made, Aes Sedai are merely altering is physical properties and therefore not creating a weapon in its entirety. Perhaps this is due to the conception that examples of Cuendillar are primarily pieces of art or tableware.
After discussion with Siuan Sanche, Egwene al'Vere comes to agree that the Three Oaths are the heart of what it is to be Aes Sedai. She has also openly discussed the possibility of releasing Aes Sedai from the oaths upon retiring, letting them live out their full lifespans in peace.