The trio of former traders concerned are Richard Usher, formerly from JP Morgan, Rohan Ramchandani, formerly from Citigroup, and Chris Ashton, formerly from Barclays.
Meanwhile, Matt Gardiner, formerly from UBS, has been working with prosecutors to help build the case.
The news that charges were in the pipeline was first reported by Bloomberg.
In 2015, the US authorities dished out mega fines to JP Morgan, RBS, Citigroup and Barclays after they pleaded guilty to forex rigging charges. UBS, meanwhile, was granted immunity from prosecution in the forex case, but the bank's actions were found to be in breach of a preexisting non-prosecution agreement.
The UK's Serious Fraud Office investigation into manipulation of the foreign exchange market was dropped in March of last year, with the fraud squad citing "insufficient evidence for a realistic prospect of conviction".
In the US, former Barclays and BNP Paribas trader Jason Katz pleaded guilty to his role in the price-fixing scandal earlier this month.
Meanwhile, two HSBC executives were arrested and charged last July over fraud claims linked to a $3.5bn currency transaction.