The two now-retired DGSE (General Directorate for External Security) agents, and the wife of one of them, are being prosecuted for "acts of extreme gravity", the armed forces ministry said in a statement.
They were indicted on 22 December 2017 and two of them have since been held in custody, a judicial source told AFP.
The Quotidien television programme and the Le Monde daily reported four people are suspected of having been recruited by the Chinese authorities to spy on French foreign intelligence.
Neither the armed forces ministry nor the judicial source gave the name of the other country, and gave no details of the length of time or nature of information passed over.
The armed forces ministry said: "These acts of extreme gravity have been detected by this service, which has brought these facts to its knowledge to the Paris prosecutor."