Chief Justice Warren E. Burger, renewing his campaign to improve the quality of trial lawyers, proposed an experiment yesterday: let law students at three diverse schools devote their third and final year to skills entirely related to courtroom advocacy.

What Burger termed the three Rs of the law would be taught, along with "substantial advocacy-related learning," in the first two years of law school. Many lawyers long have thought that learning the fundamentals of law does'nt require three years, he said.

The final academic year would be 12 full months, "roughly comparable to a medical intership," in which a prospective advocate would be immersed "in every phase of hte litigation process," from initial interviews with clients to the preparation of trial briefs, Burger said.

He unveiled the proposal in his annual address, at the Mayflower Hotel, to the American Law Institute, and educational group that seeks to improve the administration of justice.

To pay for the experiment, which would have a suggested life of three-to-six years, Burger nominated "private coundations interested in the omprovement of justice, including organizations."

"I am totally confident" they "will be prepared to provide for the cost," he said.

Burger said there is "nothing novel" about the proposal, which he described as "a wedding of theory and practice." He reminded his audience that during most of the nation's first century, advocates, including some of the greatest, learned their craft largely or entirely by acting as apprentices to practitioners.

He said his proposal would be "a return to first principles and other practices with modifications essentials for the times in which we live."

He did not offer what he called "a detailed blueprint," saying that implementation should be left to law teachers "more qualified than I."