* Global-backed Geneva 2 talks scheduled for Wednesday
* Rebel brigades show support for first time - opposition
* Coalition fractured, has little influence on ground (Adds statement from United Nations in paragraphs 18-19)
By Dasha Afanasieva
SILIVRI, Turkey, Jan 18 (Reuters) - Syria's main political opposition group in exile agreed on Saturday to attend internationally sponsored peace talks, and said for the first time three rebel fighting forces also wanted to take part.
The agreement by the Syrian National Coalition - and the chance of fighters backing the process - will be a boost for Western supporters of the "Geneva 2" talks seen as the most serious global effort yet to end the near three-year conflict.
Syrian President Bashar al-Assad's government was not immediately available to comment on the prospect of rebel militia representatives playing a role at the negotiations to end fighting that killed more than 100,000 people.
National Coalition spokesman Louay Safi told Reuters the Soldiers of the Levant, the Syrian Revolutionaries Front and the Mujahideen Army all wanted "to have some representation within the delegation" at the talks on Wednesday in Montreux.
It was not immediately clear what role they might play.
Rebel brigades had previously rejected Geneva - demanding the removal of Assad before talks. Their support is seen as critical if any deals have any chance of being rolled out.
All three are established forces, through restrictions on journalists in Syria makes it impossible to give independent estimates of their size.
A fourth fighting group, the Islamic Front - thought to be bigger than the other three combined - was still deciding whether to attend, Safi added.
Al Qaeda-linked rebels, increasingly involved in the fighting, have shown no interest in a political process.
The fractured National Coalition itself has little influence on the ground in Syria.
Major Isam el Rayyes, spokesman of the Syrian Revolutionaries Front, confirmed his group was now interested.
"The Syrian Revolutionaries Front and two other major fronts want to be represented at Geneva but we will not send our brigade leaders," he told Reuters.
There was no immediate comment from the other two.
Coalition discussions to appoint a delegation were set to go into the night. Sources said meetings with the Islamic Front were also taking place in Istanbul.
Western powers had pressed the opposition to commit to the talks and on Saturday France welcomed the Coalition's decision, vowing to make sure the discussions ended up setting up a transitional Syrian government with full executive powers.
"This brave choice, despite the provocations and acts of violence by the regime, is a choice to search for a peaceful solution," French Foreign Minister Laurent Fabius said on Saturday.
U.S. Secretary of State John Kerry called it "a courageous vote in the interests of all the Syrian people who have suffered so horribly under the brutality of the Assad regime and a civil war without end."
U.N. Secretary-General Ban Ki-moon said he welcomed the participation of the opposition groups.
"I look forward to the opposition's expedited formation of a delegation that broadly represents the diversity of the Syrian opposition, including women," he said in a statement.
The Coalition decision had been delayed repeatedly as more than 40 members threatened to leave the body, and eventually shunned the vote.
One Coalition member, Khaled Khoja, told Reuters on Saturday that the vote was illegitimate and that his group was considering a formal challenge.
Out of those that did take part, 58 Syrian National Coalition members voted to attend and 14 voted against, said the group's media office. Another three abstained it added.
"It was a tough vote," the head of the Coalition's media office, Khaled Saleh, told Reuters. The Geneva 2 process would be a "political and media battle, and on balance we decided that we must fight it alongside the war on the ground," he added.
Syrian officials have announced a delegation to attend the Jan. 22 talks, though they dispute the invitation letter's focus on setting up a transitional authority, saying the priority is "to continue to fight terrorism" - a phrase they use to describe Assad's battle with increasingly radical rebels. (Additional reporting by John Irish in Paris and Louis Charbonneau in New York; Editing by Andrew Heavens and Lisa Shumaker)