The three Democratic presidential candidates, in a showdown debate three days before the New York primary, clashed briefly Saturday over the Middle East and made some of their sharpest attacks yet on the Reagan-Bush administration.

The hottest point in the one-hour nationally televised debate came when Massachusetts Gov. Michael Dukakis and Sen. Albert Gore of Tennessee took issue with Jesse Jackson's previous overtures to the Palestine Liberation Organization, which is a sworn enemy of Israel.Jackson, defending a meeting that he held with PLO leader Yasser Arafat, noted, "the pope met with Arafat as well" and contended, "as a moral leader, it was the right thing to do. I met with him and made the same basic appeal.

"If you cannot talk with your enemies, you cannot stop your enemies from being your enemies," Jackson said.

Gore and Dukakis charged that Arab leaders needed to show more concessions toward Israel, and Gore insisted the PLO must refrain from terrorism before it can be regarded as a negotiating party.

U.S. policy toward Israel is a key question in New York because Jewish voters constitute 25 percent of the Democratic electorate in the Empire State. Dukakis and Gore are vying for that bloc.

"If one of the parties fails to make the basic concession that terrorism such as blowing up school buses with innocent children on them renouncing that kind of act as an instrument of politics" then the United States should not deal with them, Gore said.

"I don't see why it's so different for Arafat or any of the other Arab leaders to get up and do what (Egyptian President Anwar) Sadat did in 1979 (in making peace with Israel)," added Dukakis, noting, "it takes courage."

After the debate, Gore said Jackson had "made a step forward" recently by distancing himself from Arafat, but added: "He has advocated a Palestinian state. I have not."

At another point in the debate, Gore, who polls show is running well behind Dukakis and Jackson, gently renewed his claim that he is the candidate with the most foreign policy experience, asking the people to decide "who of the three of us on stage has the experience and the ability to best explore if (Soviet leader Mikhail) Gorbachev is offering something new."

The comment, however, was mild and far less severe than Gore's contentions earlier in the week that Dukakis was guilty of making major foreign policy misstatements.

Meanwhile, a new poll in the Sunday editions of the New York Daily News showed Dukakis still holding a large lead in New York, but Jackson and Gore making gains.

The survey, conducted Monday through Thursday, showed Dukakis with 51 percent, Jackson with 37 percent and Gore with 10 percent.

The poll, conducted among 2,100 Democrats by Chilton Research Services of Radnor, Pa., claimed a margin of error of 4.5 percent.

During the bulk of the debate Saturday, all three Democrats generally stayed away from attacks on each other, preferring to save their sharpest words for the Reagan administration and Vice President George Bush.