The United States yesterday exploded its third and largest underground nuclear weapons test this year, emphasizing the Reagan administration's determination to continue testing in the face of repeated Soviet calls for a moratorium.

Code-named Jefferson, the explosion at the Nevada Test Site was to determine the reliability of an unidentified nuclear weapon designed by Lawrence Livermore National Laboratory in California and in production, according to government sources.

The Energy Department, which conducted the test, did not identify the weapon, but Livermore currently produces warheads for the MX intercontinental ballistic missile (ICBM), Pershing II medium-range missile and the B83 strategic bomb.

The size of the blast 2,000 feet underground was between 20 kilotons and 150 kilotons and shook the ground at least 33 miles from ground zero, a department spokesman said.

The year's first U.S. explosion March 22 tested an early design of a warhead for the proposed Midgetman mobile ICBM. The second, 13 days ago, tested effects of radiation from the device on the MX warhead and other military equipment.

In Moscow, the Soviet news agency Tass said yesterday that the "nuclear explosions in Nevada dash a unique chance to make a real beginning to the disarmament process."

Last July, Soviet leader Mikhail Gorbachev declared a moratorium on Soviet testing and invited President Reagan to join it. Reagan refused and asked the Soviets to accept new on-site monitoring procedures to verify the 1974 threshold test-ban treaty limiting explosions to 150 kilotons or less.

The equivalent of 150,000 tons of TNT, such blasts would be more than 10 times larger than the bomb dropped on Hiroshima in 1945.

Reagan invited Gorbachev to send Soviet scientists to observe yesterday's test and the workings of a portable, on-site monitoring device called Corrtex, which recorded the size of the blast. It was the third test of the portable monitoring device designed by the Los Alamos Nuclear Laboratory in New Mexico.

In a related matter, the State Department has told Congress that it believes that a total nuclear test ban could lead to proliferation of such weapons since nations dependent on the United States for their security would lose faith in reliability of the U.S. stockpile and build their own.