Three new men took their seats today on the Soviet Politburo and, in doing so, pushed the country's ruling group across the generational divide.

No longer can it be so easily said that the Soviet Union is run by a group of old men: as of today, a majority of the 13-member group will be under 65. Only four months ago, seven out of 12 Politburo members were over 70.

The shift is not simply statistical. New Soviet leader Mikhail Gorbachev, still the youngest member at 54, has moved quickly to refashion the Soviet leadership in his own image, picking younger, better-educated men, each with a background of practical, managerial experience.

While the Soviet Union in recent years has been governed by aging men, most previous leaders acted similarly to Gorbachev in bringing in their own new men and the late Leonid Brezhnev also named several Politburo members who where in their 50s. They remained, however, and are only now giving way to a younger generation.

The key thread running through Gorbachev's choices, endorsed today by the Central Committee, however, is that they, like Gorbachev, are all Andropov's men.

Yuri Andropov, during his brief tenure as Soviet leader from November 1982 to his death in February 1984, laid the groundwork for Gorbachev's first major personnel moves by guiding Yegor Ligachev, Nikolai Ryzhkov and Viktor Chebrikov to positions close to the center of Soviet power.

It was under Andropov that Ligachev, 64, made his meteoric rise, from first secretary of the Tomsk party to a position on the party secretariat in charge of personnel. Jumped into the Politburo by Gorbachev without passing through the usual probation as a nonvoting, candidate member, Ligachev is now reported to hold the post of ideological secretary as well, which would put him second behind Gorbachev in the party hierarchy.

Chebrikov's career progressed at a steadier rate. Starting in Dnepropetrovsk, a stronghold of support for Brezhnev, he came to Moscow in 1967 as chief of personnel at the KGB secret police, where Andropov was then chief. In 1982, after Andropov became party head, Chebrikov, 61, was moved into the top KGB job and, again under Andropov, became a candidate member of the Politburo in 1983.

Ryzhkov, 54, an expert in heavy industry with a background at Gosplan, the Soviet State Planning Committee, was brought into the party secretariat by Andropov, where he was given responsibility in the economic field, heading a new unit charged with integrating various economic efforts.

Like Ligachev, Ryzhkov has been vaulted straight onto the Politburo without serving as candidate member. Such rapid promotions have been extremely rare: in recent times, only Foreign Minister Andrei Gromyko and the late defense minister Andrei Grechko were elevated directly onto the Politburo.

Even Viktor Nikonov, 56, named today to the party secretariat where he is expected to take over the agricultural slot once occupied by Gorbachev, got his first big promotion under Andropov, becoming agricultural minister of the Russian Federation in January 1983.

The only exception to the pattern in today's promotions is Defense Minister Sergei Sokolov, named one of six candidate members of the Politburo. His appointment is widely regarded as honorific.

That Gorbachev could afford to break unwritten rules and reach down below the ranks of candidate members for Politburo choices is seen here as a sign of political confidence and the momentum of his leadership.

In picking his team, Gorbachev has selected expertise in areas that have already become the hallmark of his six weeks in office.

Like the men, the themes pushed by Gorbachev can be traced back to Andropov, whose tenure also saw a push toward modernization and the first infusion of new blood into the aging leadership left behind by Brezhnev. Starting from his first speech made after his selection March 11, Gorbachev has stressed the need for accountability among the party leaders and a drive to make both the Soviet economic system and its citizens work better, harder and for more tangible results.

Now those themes are expected to be backed up by people with experience suited to the task: Ligachev in personnel policy, Ryzhkov in economics and Chebrikov with the power of the KGB behind him.

Ligachev, a well-dressed, well-spoken man, spent 18 years as local party chief in Tomsk, an important industrial center in the Urals. Before that he had worked in party and government posts in Novosibirsk, in Siberia, and for four years in the early 1960s in Moscow in a bureau of the Central Committee.

Responsibility for party personnel has given Ligachev a sensitive and influential post in the hierarchy since 1983. In 1983 and 1984, he worked with Gorbachev in supervising selection of candidates.

Ryzhkov, after Gorbachev the Politburo's youngest member, made his name in Sverdlovsk in the Urals where by 1971 he headed an important manufacturer of armaments and heavy machinery.

In 1975, he came to Moscow as first deputy minister of heavy machine building and in 1979 became first deputy chairman of Gosplan.

Clean-cut with youthful good looks, Ryzhkov could pass as an Ivy League professor. Since joining the party secretariat, he has traveled widely in Eastern Europe, where experiments in matching centralized planning with incentives for local initiative could serve as models for some of the modest economic reforms under discussion here.