Lee Cheuk-yan, 69, Albert Ho, 74 and Chow Hang-tung, 41 have been behind bars since they were charged in 2021. Their sentences are expected to include time already served.
The United Nations, rights groups and foreign governments have condemned their convictions under a national security law that Beijing imposed on Hong Kong.
A court ruled in August that the activists had breached the Chinese constitution by calling for an "end to one-party dictatorship".
Lee and Chow, who pleaded not guilty, were sentenced to seven years and seven years and three months respectively, AFP journalists in court heard.
Ho, a former lawmaker who pleaded guilty in January, got five years and two months.
The verdict read that although the case did not involve violence, and the defendants had not put forward a specific timetable to carry out an "end to one-party dictatorship", their offence was still of "a serious nature".
Chow, a barrister who represented herself, told a hearing last month that she had "nothing to repent for".
She raised both arms and waved at supporters as she was escorted out by officers.
Supporters outside court hugged her mother, Medina Chow Lau Wah-chun, who said her daughter had maintained high spirits during the trial.
"She is not a competent barrister. She is simply an outstanding defendant," her mother said.
Lee crossed himself, smiled and bowed at his wife Elizabeth Tang Yin-ngor and supporters, including high-profile activist Cardinal Joseph Zen, who stood outside gripping each others' hands.
"I'm really sad, and very angry," Tang told reporters, saying Lee would "definitely appeal".
Dozens of police swarmed around the court and set up cordons holding back crowds.
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