The three members of the London-based group were arrested Monday in Ringsted, 60 kilometers (40 miles) southwest of Copenhagen, for the suspected spying on people and companies over a period of six years from 2012.
They appeared before the nearby Roskilde City Court where judge John Larsen on Tuesday ordered the hearing held behind closed doors, meaning no details were made public.
Heavily armed police officers with machine guns guarded the courthouse in Roskilde, about 40 kilometers (25 miles) west of Copenhagen. The men who can't be named under a court ban, pleaded not guilty. The men are facing preliminary charges of espionage under a milder paragraph that could give them up to six years in jail.
In the same case, another man was arrested Monday in the Netherlands — the historic Dutch city of Delft — for allegedly plotting one or more terror attacks in Iran and for membership of a terrorist organization.