Lawyers for Chris Ashton, Rohan Ramchandani and Richard Usher, who worked at Barclays Plc, Citigroup Inc and JPMorgan Chase & Co, respectively, countered that while the three had exchanged information, nothing they did was illegal.

U.S. prosecutors announced the charges in January 2017, and the three came to the United States to face them voluntarily.

In his opening statement on Wednesday, prosecutor Jeffrey Martino, of the U.S. Department of Justice, said they used a chatroom they dubbed "the cartel" to coordinate euro-U.S. dollar transactions in order to maximize their profits.

"They worked together to stack the deck," he said, harming other participants in the market, including in the United States.

Martino said jurors would see the defendants' chats and hear their calls. He also said they would get a first-hand account of the scheme from another participant, former Barclays and UBS AG trader Matthew Gardiner, who is expected to testify under an agreement with prosecutors.

Ramchandani's lawyer, Heather Tewksbury, said Gardiner was testifying to avoid being prosecuted himself. She said the prosecutors' case relied on "cherry-picked" chats and that the "cartel" moniker was a joke.

Ashton's lawyer, Lisa Manning, similarly said that the chat room went by "dozens of silly names." Usher's lawyer, Michael Kendall, said chats like the one at issue were commonplace.

"A typical trader might have dozens of chats," he said.

All three said it was not illegal for traders to share information, and that their transactions were not coordinated as part of any scheme to fix prices.

The case followed worldwide investigations that resulted in about $10 billion in fines for several large banks, and the firing of dozens of traders.

Several others have been charged in the U.S. probe, including Mark Johnson, a former head of foreign exchange cash trading at HSBC Holdings Plc who was sentenced to two years in prison in April after being found guilty by a jury, and former Barclays trader Jason Katz, who pleaded guilty and faces sentencing next year.

Barclays, Citigroup, JPMorgan, BNP Paribas, Royal Bank of Scotland Group Plc and UBS Group all entered related guilty pleas, and were collectively fined more than $2.8 billion.

(Reporting by Brendan Pierson in New York; Editing by Jeffrey Benkoe and James Dalgleish)

By Brendan Pierson