Are the women who tied Shadow to the world tree the Zorya sisters? If not, who are they and why would they help revive Laura?
No, the women at the world tree are not the Zorya sisters.
The world tree is drawn from Norse mythology, so the closest match to the figures shown from Norse mythology would be the Norns. Here is an excerpt from Wikipedia:
In Snorri Sturluson's interpretation of the Völuspá, Urðr (Wyrd), Verðandi and Skuld, the three most important of the Norns, come out from a hall standing at the Well of Urðr or Well of Fate. They draw water from the well and take sand that lies around it, which they pour over Yggdrasill so that its branches will not rot.
Gaiman is fond of the image of the triple goddess of Maiden, Mother, and Crone, the three that weave people's fate, among other things. Having used that as a character in his Sandman works, the three Norns Urðr, Verðandi, and Skuld, responsible for fate is likely another representation of this, even though there are actually many other Norns.
As for why they helped Laura... honestly, there's not much here to do but speculate. Even Loki was puzzled by this. Shadow's real name is Baldur, and I've tried to look through those myths, and there isn't really any corollary, nor does Laura have any easy fit into that tale. Possibly they simply helped her because Shadow still had things to do after he was dead, and Laura had one last part to play in his tale.
They're the Norns.
I just read these chapters yesterday, and the text of the book makes it quite clear.
Firstly, they're definitely not the Zorya sisters:
There were three women standing by the tree. At first glance Shadow thought that they were the Zorya, but he realised in moments that he was mistaken.
But they are sisters:
They looked like a set of Russian dolls: a tall one - she was Shadow's height, or even taller - a middle-sized one, and a woman so short and hunched that at first glance Shadow wrongly supposed her to be a child. Still, they looked so much alike - something in the forehead, or the eyes, or the set of the chin - that Shadow was certain the women must be sisters.
And one of them is called Urðr, meaning Past:
The biggest woman, whose name seemed to be Urtha or Urder - Shadow could not repeat it back to her to her satisfaction - told him, in pantomime, to take off his clothes.
In his delirium, Shadow became the tree. Its roots went deep into the loam of the earth, deep down into time, into the hidden springs. He felt the spring of the woman called Urd, which is to say, Past. She was huge, a giantess, an underground mountain of a woman, and the waters she guarded were the waters of time.
All quotes are from Chapter 14 of American Gods, except the final paragraph, which is from Chapter 15.
So there's clear canonical proof that Radhil's answer is correct and the three women in the field are the Norns: Urðr, Verðandi and Skuld.
Radhill and Rand have already given the answer: they are the Norns. I'd like to mention though that the answer is given explicitly in chapter 18. This is when Laura delivers the stick to Mr. World.
“[…] Shouldn't you be further along on the whole road-to-rot-and-ruin business by now?”
“I was,” she said simply. “But those women, in the farm, they gave me water from their well.”
An eyebrow raised. “Urd's Well? Surely not.”
She pointed to herself. Her skin was pale, and her eye sockets were dark, but she was manifestly whole: if she was indeed a walking corpse, she was freshly dead.
“It won't last,” said Mr. World. “The Norns gave you a little taste of the past. It will dissolve into the present soon enough, and then […]”