According to the Office for Cyberspace Administration, the social media platforms Weibo, WeChat, and Baidu Tieba have failed to adequately monitor the content users are sharing.
Users of Weibo, Tencent's WeChat and Baidu's Tieba are believed to have "spread information of violence and terror, false rumors, pornography and other information that jeopardizes national security, public safety and social order", the internet regulator said in a statement.
The timeline of the case has not yet been revealed but any subsequent progress will be announced by the CAC's branches in Beijing and Guangdong, where the three companies are based.
Weibo and Tencent did not make a comment when one was requested by email.
There has also been a tightening of surveillance leading up to the Community Party's 19 National Congress later in 2017, when world attention will be on the world's second largest economy.
Shares of Hong Kong-listed Tencent, in which Napers owns 33 percent, fell nearly 5 percent. Baidu and Weibo are listed in the U.S.
Last month, the cyber authorities called on the same companies to carry out immediate "cleaning and rectification" at a meeting with their representatives, where the authorities cited specific examples of illicit content, including rumours about party officials and misrepresenting Chinese military history. It is targeting WeChat, the immensely popular messaging app run by Tencent Holdings Ltd.,.
Prior to the meeting, Weibo was ordered to partially close its video site over violations, wiping out a combined US$1.3 billion (S$1.8 billion) worth of stock between Weibo and parent company Sina Corp.
In May, new regulations were released for online sites that have news that expanded the curbs on content, while requiring services to be overseen by an editorial staff that was party sanctioned.