The trio were among nine people arrested by the police this week over an alleged plot to make bombs and plant them at public facilities such as courtrooms, cross harbour tunnels and railways.
The force said the nine, including six secondary school students, were part of a pro-independence group called “Returning Valiant”.
Three of the defendants were each charged with one count of conspiracy to commit terrorist activities before appearing in court for a mention hearing.
The trio, aged between 15 and 19, are accused of conspiring with others, between June 6 and July 5, to organise, plan, commit, participate in or threaten to commit terrorist activities intended to cause grave harm to the society – including explosion, arson, sabotage of means of transport or transport facilities, or other dangerous activities which seriously jeopardise public health or safety.
The alleged activities are said to aim at coercing the central or Hong Kong government, or intimidate the public to pursue their political agenda.
Chief magistrate Victor So, designated to deal with national security cases, refused to grant bail to the three defendants and adjourned their case to September 1.
The youngest suspect among the trio, who is 15, will apply for bail again in court next Thursday.
The other six who were arrested have been released on bail and are required to report to the police later this month.