Joshua Wong, Alex Chow and Nathan Law were sentenced to six, seven and eight months in prison respectively.
Wong and Chow were found guilty past year of unlawful assembly, while Law was found guilty of inciting people to take part in the assembly.
Hong Kong enjoys much greater freedoms than on the mainland, with its liberties enshrined in the handover agreement made when the city was given back to China by Britain in 1997.
The bespectacled Wong, who was 17 when he became the face of the student-led democracy movement, punched his fist in the air as he left the court room and shouted: "Hong Kong people don't give up". Youthful activists brought major thoroughfares to a standstill for 11 weeks to protest Beijing's plan to restrict elections in the Chinese-ruled former British colony.
Wong and Law were originally given community service and Chow had received a suspended three-week prison sentence.
They were immediately taken to serve their sentences of up to eight months, which have the added outcome of blocking each of them from seeking public office for five years.
Having been tried and found guilty of unlawful assembly past year, Wong was sentenced to community service.
As Mr. Wong put it to a reporter from The New York Times before his sentencing: "The government wanted to stop us from running in elections and directly suppress our movement".
About 100 supporters later swarmed the prison vans taking the three away from court, shouting "shame on political prosecution" and waving yellow umbrellas, a symbol of the city's pro-democracy movement, a Reuters witness said.
Ahead of the judgement today, the department of justice said there was "absolutely no basis to imply any political motive" on its part in relation to the case.
Prior to the sentencing, Wong urged his supporters who had gathered in the High Court lobby to continue fighting for full democracy.
In recent months, dozens of protesters, mostly young people, have been jailed for their roles in various protests, including a violent demonstration that the government called a riot in early 2016.