All six were among seven men arrested last week in central England after a search of an impounded vehicle turned up hidden guns and other weapons, prompting police to trace and arrest the driver, the passenger and other suspects.
Authorities have said the case is not linked to the upcoming 2012 London Olympic Games.
The six men all were charged with preparing an act of terrorism, which carries a potential life sentence. The seventh person arrested in the case—a 43-year-old man—was released Tuesday night without charge, police added.
West Midlands Police did not name the latest three, but said late Tuesday they were two 22-year-olds and a 24-year-old.
The first three men appeared at London's Westminster Magistrates' Court earlier Tuesday. Jewel Uddin, 26, Omar Mohammed Khan, 27, and Mohammed Hasseen, 23, spoke only to confirm their names and ages at the brief hearing, and were told they will be kept in custody until another hearing on July 31.
All three men, from the mostly Muslim neighborhood of Sparkhill in the central England city of Birmingham, did not enter any pleas or seek to be freed on bail.
Prosecutor Piers Arnold told the court the men allegedly were making preparations to attack a June 30 march of the anti-immigrant English Defense League in Dewsbury, central England. He did not say whether the suspects' alleged plans failed or if they decided not to carry them out.
Arnold said police seized a Renault Laguna car on the M1 highway near Sheffield, in central England, on June 30 after the rally and a search of the vehicle two days later found two sawed-off shotguns, ammunition, knives and an improvised explosive device stashed in the trunk.
The English Defense League claims to be a peaceful opponent of radical Islam. Its opponents accuse the party of racism, however, and its protests have often turned violent.
West Midlands Police said the men charged late Tuesday will appear Wednesday morning at Westminster Magistrates' Court.