Joshua Wong, Nathan Law and Alex Chow were jailed in August last year for their role in the 2014 Umbrella Movement mass pro-democracy protests, after Hong Kong's government pushed for harsher sentences.
A lower court had originally given Wong and Law community service orders and Chow a suspended sentence. But after the government's intervention they were jailed for between six and eight months by the Court of Appeal.
The government's move was seen as further evidence of Beijing's growing influence over the semi-autonomous city, with Chinese authorities particularly riled by the emergence of activists calling for independence for Hong Kong.
Wong and Law's political party Demosisto wants self- determination for the city.
Also Read
"Now is the chance for us to be aware how the courts of Hong Kong will recognise, will position, the motivation of civil disobedience," Wong, 21, told reporters ahead of the hearing at the city's court of final appeal.
Wong said even if he was sent back to prison, he believed Hong Kong people would still keep fighting for democracy.
Their arrests sparked wider rallies which exploded two days later when police fired tear gas at the crowds, triggering the Umbrella Movement rallies which brought parts of the city to a standstill.
Defence lawyer Robert Pang warned that harsher sentences had a "chilling effect" on young people expressing their views.
"We must guard against discouraging young, idealistic persons who are doing this not for their personal benefit...from expressing their point of view," Pang told the court.
The Umbrella Movement called for fully free leadership elections to replace a system where the city's chief executive is selected by a pro-Beijing committee, but failed to win any concessions.
Since then there have been growing signs that China is increasing its control over Hong Kong.
Hong Kong has been governed under a "one country, two systems" deal since 1997, when Britain handed the territory back to China.
The deal allows citizens rights unseen on the mainland, including freedom of speech and a partially directly elected parliament, as well as an independent judiciary.
Wong also faces a possible jail sentence tomorrow for obstructing the clearance of a major protest encampment in 2014, a charge to which he pleaded guilty.
Disclaimer: No Business Standard Journalist was involved in creation of this content