The three-man military junta that governs Argentina named former Army chief Gen. Roberto E. Viola today to assume the presidency for a three-year term beginning next March.

Incumbent President Rafael Videla, the general who brought the military to power in a 1976 coup, is to remain in office until March 29.

The Argentine radio station Radio Rivadavia interrupted a scratchy tango at 1 p.m. with the official announcement that leaders of the armed forces "agreed that regardless of their different approaches, the more important interests of the country's institutional future had to take precedence."

That was almost certainly a reference to rumors of a power struggle that have been circulating all week.

Although the announcement had for many months been scheduled for Sept. 29, the junta last Tuesday said it had postponed the decision until Oct. 9. This reportedly was because the Army, Navy, and Air Force commanders were still arguing over the division of ministries, governorships, economic policy and the amount of power Viola will wield as president.

Since the date has been pushed back, the timing of today's announcement came as a surprise -- but not the choice.

Viola, 55, a career Army officer, has a reputation as a politically cagey "moderate." He is said to have overcome harder-line military men in bringing a gradual slowdown to the violent state retaliation against guerrilla terrorism.

A year ago, aftaer the release into exile of newspaper editor Jacobo Timerman -- who had been held two years without trial for alleged leftist connections -- an enraged Army regional commander demanded ouster of Viola as commander-in-chief and threatened to use his troops to back up the demand.

Viola sacked the rebellious general and resolved the crisis without bloodshed.

Nevertheless, Viola has publicly supported the government declaration that subversion and subversive ideas must be cleared before Argentina's electoral process can resume.

Observers from Amnesty International and the Inter-American Commission on Human Rights have documented thousands of cases of kidnapping, torture and murder by security forces during Viola's 1978-79 tenure as Army commander-in-chief.

He is known as a strong advocate of the controversial program of Economy Minister Jose Martinez de Hoz, who has directed a massive "opening" of a statist economy. Many Argentines say angrily that he is more powerful than then president. Viola, it is said, wants more flexibility in determinng how the economic policy will be applied.

Adopting a low profile in recent months, Viola is in keeping with the current government effort to keep "personalism" -- a thinly veiled reference to the enduring populist charisma of the late president Juan D. Peron -- out of the presidency.

Viola is described as a heavy smoker and avid newspaper reader, a properly Argentine aficionado of soccer and the tango, and a family man who refuses to be disturbed during his weekends at home.