Three Democratic presidential contenders yesterday accused the Reagan administration of promoting dangerous nuclear exports and endorsed legislation to clamp down on such shipments.

Former vice president Walter F. Mondale and Sens. Alan Cranston (D-Calif.) and Gary Hart (D-Colo.), as well as six House Democrats, fired off a sharply worded joint letter to President Reagan, saying his administration is exploiting loopholes in existing law and "opening the door for exports of the very technologies and materials that can be turned into weapons of mass destruction."

The three other Democratic presidential hopefuls, Sen. John Glenn of Ohio, Ernest F. Hollings of South Carolina and former Florida governor Reubin Askew, declined to endorse what backers described as a "major initiative" to combat nuclear proliferation.

The drive included introduction in both the House and Senate of a proposed Nuclear Explosives Control Act to impose sharp new restrictions on worldwide trafficking in materials such as plutonium and highly enriched uranium.

Rep. Richard L. Ottinger (D-N.Y.), the principal sponsor in the House, said at a crowded news conference that the bill also contains "a major innovation" in that it would offer cooperating nations a discount price on U.S. uranium enrichment services if they agree to use low-enriched supplies "unsuitable" for nuclear weapons.

Hart, the principal sponsor in the Senate, said that 44 metric tons of separated plutonium--enough to make 6,500 bombs of the size that destroyed Nagasaki--already exist "in nuclear power programs throughout the Free World." (A metric ton contains about 2,200 pounds.) By the year 2000, he warned, that could grow to 600 tons--enough for 88,000 weapons--if plutonium use continues.

Cranston, who is making the arms race the overriding issue of his campaign, put the blame directly on Reagan, saying that the president has "encouraged the use of plutonium" and has shown himself "insensitive to the national security implications of his proliferation policies."

" . . . . When it comes to controlling the spread of nuclear weapons to still more nations, Ronald Reagan's policy is a radical policy," Cranston charged. "He's listened more to the nuclear salesmen than to the nonproliferators."

Mondale signed the five-page letter to Reagan deploring the "dangerous stance," but did not attend the briefing. Advance billings had said that Glenn and Hollings would be there, but neither showed up.

A spokesman for Glenn said he "chose not to sign on" because "he doesn't agree with all the specifics in the bill or in the letter," such as its complaints about a pending arrangement with Japan for plutonium extraction.

Hollings' office said he stayed away because "you don't get nuclear nonproliferation by unilateral action on the part of the U.S." Askew issued a statement saying he feared that the tart letter to Reagan would make nonproliferation "a partisan issue."