Eastern Court on Friday found Demosisto members Isaac Cheng, Ho Sau-yee and Ng Ka-yi guilty of failing to behave in an orderly manner during a public hearing at Legco in March last year.
The trio were fined HK$1,000 each for staging a protest and dashing to the chairman's seat during a hearing on the National Anthem Bill.
Magistrate Ho Chun-yiu said their actions had affected the dignity of Legco and briefly interrupted the meeting.
But he deemed the witness accounts of three Legco security guards to be unreliable, citing their inconsistencies and the contradictions between what surveillance footage showed and their versions of events.
He said one security guard's account about Ho Sau-yee jostling him with her shoulder and causing him to fall was "exaggerated and inaccurate".
Acquitting her of an additional charge of a common assault, the magistrate said CCTV footage didn't show any physical contact between the guard and the activist and she had instead tried to avoid the guard, who had rushed at her at one point.
The magistrate said considering the behaviour of the Demosisto trio had caused only a mild interruption and that they were not acting out of self-interest, he decided not to impose prison terms, adding that the three, all university students, would be pillars of society in future.
After the hearing Cheng said he was disappointed with the court decision as it shows the government is using the law as a weapon to crush dissent and the courts are unable to protect people's rights in the face of this challenge.
He said the legal costs of protests will only be higher once the planned national security law is passed, and he urged people to take part in a referendum organised by a pro-democracy coalition this Sunday, which will decide whether to hold a general strike over the imminent legislation.
"After the national security law, actually my prosecution will be affecting national security. So the prosecution will be much more serious. So we urge the Hong Kong people to come out and fight against the government and fight against this dictatorship regime and to take part in this upcoming public voting."