Liu Ping, Wei Zhongping and Li Sihua were charged with "illegal assembly" for unfurling a banner outside a housing complex in April, calling for government officials to declare their assets, lawyers said.
Asset disclosure is seen by some as a key change that could help China prevent graft after reports of enormous wealth amassed by officials and their families.
The court in in Xinyu, in the central province of Jiangxi, was forced to suspend the hearing after the defendants withdrew permission for their lawyers to represent them in protest at what they called the court's "illegal conduct", said Zhang Xuezhong, who had been part of Liu's defence team.
Zhang previously told AFP that one witness had been detained, while others were not approved to appear at the trial.
China's new leadership under President Xi Jinping has touted a campaign against corruption, with some high-ranking politicians investigated.
But no systemic changes have been implemented and the Communist Party, which maintains tight control over the political process, is highly intolerant of organised dissent.
Defence lawyer Pang Kun told AFP that he had been detained by authorities for several hours yesterday, alongside a writer who planned to witness the trial who was still being held today.
Court officials could not be reached for comment by AFP.
Liu, 48, an unemployed former steel worker, was associated with the "new citizens movement", a loose grouping of activists calling for reforms to China's legal system, her daughter Liao Minyue said.
At least 15 others involved in the group have also been detained in recent months, according to US-based rights group Human Rights Watch (HRW).
Activists associated with the movement who have been formally arrested include prominent lawyer Xu Zhiyong, and billionaire investor Wang Gongquan.