The chief of staff of Guatemala's ruling Army was shot to death last night, the third major assassination this year in the most populous and chronically the most violent Central American republic.

Unidentified gunmen in two cars opened fire with automatic weapons as Gen. Jose Cansino, 52, was being driven in an unescorted car through the city center. His chauffeur and bodyguard also were killed and two bystanders were injured.

Cansino was third-ranking of the ruling generals, following the president, Gen. Romeo Lucas Garcia, and the defense minister, Gen. Otto Spiegler Noriega. Vice President Francisco Villagran Kramer, a moderate politician elected with Gen. Lucas in an abortive opening to the left, wields little authority in the government.

The victims of two earlier assassinations were opponents of the military: former foreign minister Alberto Fuentes Mohr, killed in January, and former Guatemala City mayor Manuel Colom Argueta, gunned down in March.

In both cases the assailants acted in daylight within the city center. There has been no announced results of promised investigations nor have any of the violent extremist groups in the nation of almost 6 million people claimed responsibiltiy.

Current factional violence dates to the military ouster, with the aid of the Central Intelligence Agency, of an elected leftist government in 1954.