The detainees, including Patrik Jordan Mathews, a 27-year-old Canadian national who entered the US illegally last year, are suspected members of The Base, which authorities describe as a "racially motivated violent extremist group", Xinhua news agency reported citing the US Attorney's Office in Maryland as saying on Thursday.
The other two men are Brian Mark Lemley Jr. (33) and William Garfield Bilbrough IV (19). Both are Americans.
The arrest came just after Virginia Governor Ralph Northam declared a state of emergency and banned firearms on the Capitol grounds in Richmond in anticipation of the gun rights demonstration next week.
"We have received credible intelligence from our law enforcement agencies that there are groups with malicious plans for the rally that is planned for Monday," Northam said on Wednesday.
Earlier this month, Lemley and Mathews "purchased approximately 1,650 rounds of 5.56 mm and 6.5 mm ammunition; travelled from Delaware to a gun range in Maryland, where they shot the assault rifle; and retrieved plate carriers (to support body armour) and at least some of the purchased ammunition from Lemley's prior residence in Maryland", the Attorney's Office said.
Lemley and Mathews could each face a maximum of 10 years in prison if they're convicted of "transporting a firearm and ammunition in interstate commerce with intent to commit a felony offence", it added.
In addition, both Lemley and Bilbrough could face a maximum sentence of five years for allegedly transporting and harbouring Mathews.
The Base's members use encrypted chat rooms to discuss their supremacist agenda, the FBI said.
According to court documents, the extremists frequently discuss topics such as "recruitment, creating a white ethno-state, committing acts of violence against minority communities (including African-Americans and Jewish-Americans), the organization's military-style training camps, and ways to make improvised explosive devices".
( With inputs from IANS )