text: Pham Minh Hoang had written articles that "blackened the image of the country", according to the judge at the trial in Ho Chi Minh City.
  Hoang, 56, admitted having joined an opposition group but said his writings were not aimed at overthrowing anyone.
  Human rights activists say dozens of government critics in Vietnam have been given long jail terms in recent years.
  France's foreign ministry had earlier voiced "serious concern" about Hoang's case.
  His family has said he will appeal against the sentence.
  His wife, Le Thi Kieu Oanh, said she was "utterly shocked" by the court's decision.
  "I am losing hope... but as a wife, I will fight until the very end," she told the BBC's Newshour programme.
  Hoang, who was said to have written 33 articles under the pen name Phan Kien Quoc, asked for leniency, saying he did not know he was breaking the law.
  "My writings were not aimed at overthrowing anyone," he told the court. "I only pointed out the negative things in society, and I think the country needs to be more democratic."
  His lawyer said Hoang was only exercising his right to free speech, and drew from his experience in France "where there is a civilised education environment".
  Subversion has become an increasingly common charge brought against democracy activists in Vietnam - resulting in sentences of up to 15 years in prison, says Nga Pham of the BBC's Vietnamese service.
  But Hoang received the more lenient jail term of three years for a number of reasons; he is a French citizen, and although he confessed to belonging to the banned opposition group Viet Tan, he said he was not acting under its instruction.
  Viet Tan is a California-based pro-democracy group that Vietnam considers a terrorist organisation. The US has said there is no evidence to suggest that Viet Tan is a terrorist group.
target: A French-Vietnamese dissident blogger has been jailed in Vietnam for three years for attempted subversion.