Former Treasury secretary Robert E. Rubin, one long leg elegantly draped over the side of a chair, casually began another chapter in his own "Master of the Universe" story today as he became one of three chairmen of Citigroup.

Sitting in the Lexington Avenue offices of the world's largest financial services company alongside Citigroup's already power-sharing co-chairmen, John S. Reed and Sanford I. Weill, Rubin seemed right at home. Though he has never run a retail bank, he will share the title of chairman of the consumer banking behemoth with Reed and Weill.

Rubin is a tremendous catch, bringing access to financial institutions and governments across the globe.

Rubin, 61, who left Treasury and returned to his Park Avenue home in July, had entertained a number of offers from Wall Street. He gently declined in an interview today to say what the other proposals were.

"When I left Treasury, I knew what I wanted to do," Rubin said. After filling up yellow legal pads with pros and cons of various positions--a Rubin trademark--he decided between tennis games this weekend to go with Citigroup. "It's an enormously broad institution," he said.

In April 1998, Reed and Weill began sharing the top title after the merger of Citicorp and Travelers Group Inc., bringing under one financial umbrella such services as consumer banking, credit-card operations, investment banking, insurance and brokerage businesses. At the time, some management experts expressed doubt that the brash, ambitious "just call me Sandy" Weill and the intense, reserved Reed would be able to work together as a team.

They admitted today that it was not easy at first, but they said they have developed a management style that works and they expressed the belief that Rubin's presence would facilitate that teamwork.

"I think all large organizations are in fact run by teams," said Reed. "The only difference here is the titles. . . . These are big companies and nobody's smart enough to be singular in their judgments."

Citigroup, which earned $7.3 billion in the first nine months of the year, is one of six or more financial institutions that could become the No. 1 global bank in the new world of financial deregulation, said economist Allen Sinai, chief global economist at Primark Decision Economics.

"It's potentially the most powerful financial institution in the world," running alongside Chase Manhattan Bank, Merrill Lynch & Co., American Express and others, Sinai said. "My instinct is Mr. Rubin is up to the challenge" of running such an institution, he said.

But Rubin denied that he has any interest in being the single chairman of the company if Weill, 66, and Reed, 61, should retire before him.

"I take what he said at face value," said banking analyst Judah Kraushaar, of Merrill Lynch. "If you're Robert Rubin and you've had this tremendous track record at Goldman Sachs and Treasury, being behind the scenes at Citigroup could potentially be the best of all worlds."

Rubin's Washington fan club was all smiles today. Federal Reserve Chairman Alan Greenspan, who had a weekly lunch with Rubin when he was Treasury secretary, yesterday called Rubin "a good friend" and said, "I wish him well in his new venture."

Lawrence H. Summers, Rubin's successor at Treasury, said: "Bob Rubin is a remarkable leader and I wish him well in his new endeavor."

Rubin will have what Reed called "the best job in the company." While he will work with Reed and Weill on strategy and overall direction of the company, Rubin will not be an executive officer with line responsibilities at the company, as the other two are.

Citigroup named Rubin to its board, where he will be chairman of the executive committee.

"How you could negotiate a deal where you have no responsibilities, but are able to participate fully, I haven't figured out," Reed said.

Rubin, who was born in Manhattan, received a degree in economics from Harvard before dropping out of its law school. After attending the London School of Economics, he came back to the United States and earned a law degree from Yale.

After two years of practicing law, he began a job search on Wall Street and landed at Goldman Sachs & Co., where he spent his career before going to Washington.

At a news conference today, there was a seeming love fest as Weill and Reed beamed before the press and some Citigroup employees in welcoming Rubin into the fold.

"We feel honored," Reed said.

Weill described Rubin as "someone I looked up to and respected on Wall Street . . . and in politics."

"I wish I'd heard those remarks, Sandy, before we discussed compensation," Rubin said.

Staff writer George Hager and staff researcher Richard Drezen contributed to this report.