Islamic extremists in Lebanon stepped up their pressure on France's Socialist government today, just two days before crucial legislative elections by releasing videotapes of three Frenchmen kidnaped in Beirut.

The release of the videotapes, in which the victims plead for a change in France's Middle East policies, came as French politicians of both left and right made their final appeals to the electorate. The official election campaign ends at midnight tonight, leaving a day of "reflection" before polling on Sunday.

Although opinion polls have been banned for the last week of the campaign, French political analysts are virtually unanimous in predicting that the moderate right-wing opposition parties will win an overall majority in the 577-seat National Assembly. This could lead to a political test of strength between a new right-wing government and Socialist President Francois Mitterrand, whose seven-year term does not expire until 1988.

France's state-run television networks refrained from broadcasting the full videotaped messages of the hostages, arguing that to do so would play into the hands of the Islamic Jihad organization, which has asserted responsibility for the kidnapings. Instead, viewers were shown brief visual shots of the kidnap victims with a voice-over commentary in place of the sound on the tapes.

Reading prepared statements, the hostages called on France to withdraw its military and economic support for Iraq in the 5 1/2-year-old Persian Gulf war. France long has been the leading western arms supplier to the Iraqi government of President Saddam Hussein.

"We are here because our country sent arms to Iraq and because France chose Saddam Hussein's side instead of remaining impartial," said Jean-Paul Kauffmann, a journalist for a French weekly magazine who was kidnaped in Beirut last year along with two French diplomats and a researcher, Michel Seurat. Earlier this week, Islamic Jihad released photographs of what it said was Seurat's corpse.

A separate and previously unknown Islamic group calling itself the Revolutionary Justice Organization said today that it had kidnaped a four-man French television crew in Moslem west Beirut six days ago. Supporting its assertion with photostatic copies of the identity cards of two of the captives, the group also denounced French policies in the Middle East.

In an Arabic statement delivered to the Visnews television agency in Beirut along with the 6-minute, 30-second videotape, Islamic Jihad said it was giving the French government a "last chance" to meet its demands. These demands include the release of five Arab terrorists serving lengthy prison sentences in French jails for the attempted assassination of former Iranian prime minister Shahpour Bakhtiar in July 1980.

The statement said the kidnapers "have not met and will not meet with any official or unofficial envoy" unless the French government moves toward acceptance of their demands. French Prime Minister Laurent Fabius has insisted that France will not give in to "blackmail" from the kidnapers.

An unofficial French emissary, Lebanese-born Dr. Razah Raad, was expected to arrive in Paris Saturday after several days of secret negotiations in Beirut and Damascus that, he said, included contacts with representatives of the kidnapers of the Frenchmen seized last year. He told a French radio station tonight that he would submit a plan to the French government that could lead to the release of the hostages after the elections.

With French politicians maintaining public silence on the hostage affair, the debate in the last hours of the official campaign was dominated by the prospect of a left-wing president "cohabiting" with his political opponents. In their final campaign speeches, several Socialist leaders warned of political instability in the event of a right-wing victory.

"Between President Mitterrand and a right-wing prime minister, the choice risks being limited to a sad alternative between a quarrel and humiliation," Fabius said at a rally in the southern city of Toulouse.

After first indicating that Mitterrand was determined to remain in office whatever the result of the election, the president's advisers have been hinting recently that he may decide to resign if stripped of effective political power. Most right-wing leaders, however, have dismissed the leaks as a political ploy aimed at scaring voters.

Since the Fifth Republic was founded by Charles de Gaulle in 1958, with staggered terms for the president and the assembly, the party in control of the assembly always has controlled the presidency.

Neo-Gaullist leader Jacques Chirac, widely mentioned as the most likely prime minister if the right wins a comfortable majority in the assembly, said the president did not have the right to open up a debate on the constitution.

"Public opinion must not get the idea from the useless debates that our constitution is not sufficiently solid to ensure the continuity of the state. If everybody respects the rules of democracy and of the republic, everything will happen in a dignified and efficient way," Chirac said.

Another leading Gaullist, Charles Pasqua, said that the right had no intention of "humiliating" Mitterrand. But he added: "If the president wants to leave, we are not going to lie across his doormat to stop him."