Tsang Kin-shing and Tang Sai-lai from the League of Social Democrats, and former district councillor Chui Chi-kin, face charges including organising and taking part in the rally that took place a day after the national security law came into force, and inciting others to join it.
Citing risks to public health and the potential for violence, police banned the annual march planned by the now-disbanded Civil Human Rights Front.
Thousands turned up that day despite the ban and 370 people were arrested.
Judge Amanda Woodcock adjourned the hearing to October 7 and 8, when the trio are expected to enter pleas officially.
Five others are jointly charged in the case. They are Figo Chan of the Civil Human Rights Front, former lawmakers Chu Hoi-dick, Wu Chi-wai and Leung Kwok-hung, and district councillor Chan Wing-tai.