Media mogul Jimmy Lai, 73, pleaded guilty at the district court on Wednesday to having knowingly taken part in an unauthorized assembly on August 31, 2019.
Former lawmakers Lee Cheuk-yan and Yeung Sum also admitted to having joined an illegal anti-government protest that year.
"I plead guilty," Lee said, but claimed that he had "done no wrong in affirming the rights of people to peaceful procession and I believe history will absolve me."
Sentences in their cases are to be handed down later. The maximum possible punishment is five years in prison.
Lai, who was arrested in August last year, had already been charged with illegal assembly and intimidation but was free on police bail.
The media tycoon, also a British citizen, has been a frequent visitor to Washington, meeting figures such as former secretary of state and ex-CIA chief Mike Pompeo.
China has accused him of treason.
Hong Kong has been governed under the “one country, two systems” model since the city, a former British colony, was returned to China in 1997.
Last year, the city enacted a new national security law, criminalizing sedition, secession, and subversion against the mainland.
Some opposition lawmakers openly called for Western intervention to stop the law from being enacted in the city and propagated in favor of secession from mainland China.
The United States and Britain actively supported the protest leaders and attempted to stir anti-China sentiments in the city.
Beijing has slammed the UK and the US for their interference in the affairs of Hong Kong as an internal issue of China.