The violence at the world heritage site was a brawl among employees, Beijing police said in a post on their verified account on Sina Weibo, a Chinese equivalent of Twitter.
Police identified the assailant as a 49-year-old Beijing resident surnamed Zheng and said he was taken to hospital after trying to kill himself following the knifing.
The official Xinhua news agency cited museum sources as saying no tourists were hurt. No details of the trio's jobs were available.
The Forbidden City, the sprawling former home of China's emperors, hosts more than 14 million visitors annually, the museum's director recently told the Hong Kong-based South China Morning Post.
China has a low murder rate, although there have been occasional previous incidents at tourist landmarks.
In August 2008, one day after the opening of the Beijing Olympics, an American businessman was killed and his wife and a tour guide critically wounded in a knife attack at the city's historic Drum Tower.
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