The United States had urged Chinese authorities to release the men immediately.
"He was bleeding right from the middle of the head", the current worker said.
In comments to the AP, the Huajian Group declined to respond to specific questions, but broadly denied all allegations, calling them "completely not true to the facts, taken out of context, exaggerated". The factories produced Ivanka Trump shoes, among other brands.
They gathered video, pictures and evidence that showed that workers at the factories were exploited, with long working hours and ill treatment by management, China Labor Watch said.
The three were investigating claims that Huajian Group factories in Ganzhou and Dongguan paid low wages and forced employees to work excessive overtime hours. Huajian had said the rights group has "undertaken illegal actions in China to gain twisted information, in order to profit". The labour group said two were taken away from a hotel room while the third was detained by customs officials in the southern city of Shenzhen while en route to Hong Kong.
The U.S. State Department called for the activists to be freed earlier this month, which China rebuffed, saying the case was an "internal matter". According to China Labor Watch investigators, until recently, workers might get only two days off - or less - per month. At the time, Hua Chunying, spokeswoman for China's Ministry of Foreign Affairs, said the men had been accused of using secret recording devices to disrupt normal commercial operations and would be dealt with under Chinese law.
The organization says the factories made shoes for the Ivanka Trump brand, along with Marc Fisher, Kylie + Kendall, Coach and other brands.
Huajian, which manufacturers shoes for a wide range of worldwide brands, has denied allegations of excessive hours and underpayment in its factories.
The brand of Ivanka Trump does not produce directly in China, but has entrusted a part of its production to companies that manufacture in this country of clothes, shoes or handbags.