A hearing for the three in federal custody is slated for Thursday, reports CBS Station WFOR in Miami.
One unidentified man was being sought and more arrests were expected as the probe widened, investigators said. FBI spokesman Mike Fabregas said the investigation was continuing. The operation emerged at a difficult time in the Northern Ireland peace process, with a pact to end three decades of sectarian and political violence foundering in part because of the IRA's refusal to disarm.
The IRA, fighting to end British rule of the province, has observed a cease-fire for months but the guerrilla group blames Britain and politicians from the Protestant majority for the stalemate over disarmament.
Fabregas said south Florida was not known as a "hotbed of support" for the IRA.
But on Monday, law enforcement agents arrested Belfast-born Anthony Smyth, Siobhan Browne, originally from Cork, and Conor Anthony Claxton, also Irish, in Fort Lauderdale on charges of illegally exporting guns through the U.S. mail.
The weapons were bought from a local gun dealer.
"The investigation to date has revealed that the above listed individuals have purchased numerous weapons, magazines and ammunition in south Florida and have mailed them, or attempted to mail them to Ireland," said an affidavit signed by Federal Bureau of Investigation agent Mark Hastbacka.
"The packages mailed have been falsely labeled as containing toys, computers, etc."
The ring was uncovered after British authorities at West Midland International Airport on July 6 X-rayed a package mailed from Florida and discovered guns. Several other packages marked as containing baby clothes and videos were also found to contain weapons, including a .357 Magnum revolver.
The Magnum was traced to Big Shot Firearms in Fort Lauderdale. Owner Edward Bluestein told investigators Browne and Smyth had first approached him at a gun show in April and bought five firearms.
Later Browne told Bluestein she was looking "for someone she could trust" and was interested in buying as many guns as he could supply, according to the affidavit.
A hand-written list faxed to him included demands for "any full auto sub-machine guns ... the smaller the better" and "anything silenced .25 and up."
Between April 29 and May 6 they bought six shotguns and about 26 handguns, including Israeli-made pistols, it said.
Bluestein told Reuters on Tuesday his dealings were legal.
"I definitely had no idea about anything. This was a short-lived thing, month or two, and I have not spoken to her in months," he said. "I would rather not speak on anything specific but it as a legal transaction. I am cooperating."
While U.S. agents were watching the suspects, Claxton mailed packages to Ireland on at least three other occasions from post offices in south Florida which were intercepted in Britain, the affidavit said.
Browne, age about 34, Smyth, 42, and Claxton, 26, have been charged with illegal exporting weapons and conspiracy. They are being held without bond and were scheduled to appear at a pretrial detention hearing in Fort Lauderdale on Thursday. They face up to 20 years in prison if convicted.