Gardaí had been monitoring the movements of the three men after they were seen acting suspiciously in various parts of Co Clare in recent weeks.
One of the men is from Limerick city, another from Cork and the third from Clare. Two are in their 40s and another in his late 30s.
The van they were travelling in was intercepted by a number of mobile armed garda units at around 9.30am yesterday.
A handgun, a sawn off shotgun, wigs and a quantity of ammunition and wigs were found by gardaí in the Dublin-registered van at Dromline on the old Limerick to Newmarket on Fergus back road.
High visibility jackets were also found in the van and a large quantity of other ‘equipment’.
“These arrests are very significant in the fight against serious crime,” Chief Supt Liam Quinn said.
The three are being held at Ennis and Shannon Garda stations under Section 30 of the Offences Against the State Act. They are believed to be experienced and hardened criminals.
The van has been removed to Ennis Garda station where it is being forensically examined.
Gardaí were refusing to speculate on what the men were up to at the time of their arrests.
However, one source believed they were on their way to carry out a major robbery in the Ennis area. A post office may have been a possible target, according to the source.
Local gardaí were backed up by armed detectives from the National Bureau of Criminal Investigation and the National Surveillance Unit.
In a separate development, Ennis Gardaí seized 10kg of cannabis with a street value of €70,000 in the town on Thursday night. One man has been arrested.