Amid the frantic diplomatic efforts to resolve the crisis in Lebanon, Prime Minister Menachem Begin's ruling coalition was strengthened substantially today by the addition of three hard-liners who oppose any Israeli concessions in the occupied West Bank and Gaza Strip.

At a meeting in his office this morning, Begin signed an agreement that will incorporate into his ruling Likud bloc the three members of the Israeli Knesset (parliament) who now represent the right-wing Tehiya Party, which was formed in 1981 by opponents of the Camp David peace accords. The agreement must be approved by the Israeli Cabinet and the Knesset, but this is considered a formality.

The addition of the three will give the coalition 64 votes in the 120-member Knesset, the largest majority the Begin government has enjoyed since the 1981 national elections here. Thus, when U.S. special envoy Philip C. Habib arrives in Jerusalem next week for talks on the Lebanese crisis, he will find Begin in a far stronger position politically than before Israel's invasion of Lebanon.

This in turn, political analysts said, will strengthen Begin's ability to resist American pressure for concessions as part of an agreement for the peaceful withdrawal of the Palestine Liberation Organization forces trapped in West Beirut by the Israeli Army.

"This puts him in a stronger position vis-a-vis the United States," one Israeli analyst said. "It is a message to the United States. Don't fool yourselves. We are strengthening the hawkish side of our government."

Beyond the immediate problem of the trapped PLO forces, the incorporation of the three hard-liners into the Israeli government undoubtedly will complicate any effort to renew the talks on granting autonomy to the Palestinian residents of the West Bank and Gaza as called for in the Camp David accords.

Reagan administration officials have spoken optimistically about the prospects of reviving the stalled autonomy talks once the PLO forces are removed from Lebanon. In any event, these officials have said, the United States has not given up hope that the Camp David formula will succeed, suggesting that a major diplomatic effort to renew the talks could come from Washington following a resolution of the crisis in Lebanon.

The Tehiya faction by itself could not block Israeli concessions in the autonomy talks. But its entrance into the government now seemed a clear sign that the Begin government is hardening its position on the autonomy issue at the same time that it is declaring that the destruction of the PLO forces in southern Lebanon has presented a new opportunity for a U.S. initiative to revive the Middle East peace process.

Tehiya was formed before the 1981 elections by the most vehement Israeli opponents of the Camp David peace accords. Rather than supporting any plan for Palestinian autonomy in the West Bank and Gaza, its members favor the outright annexation of the territories, occupied by Israel since the 1967 Arab-Israeli war.

Tehiya bitterly opposed the Israeli withdrawal from the Sinai Peninsula last April as called for by the Egyptian-Israeli peace treaty.

Having lost on that issue, Tehiya's main objective now is preventing any Israeli concessions in the West Bank and Gaza. Its leaders have pledged to play the role of "watchdogs" of the government on this issue. As part of the negotiations leading to its entrance into the government coalition, the faction won from Begin a pledge that additional funds would be budgeted by the government for the expansion of Jewish settlements in the West Bank.

The addition of three new members to its coalition at this time is clearly a side benefit to Begin from the invasion of Lebanon, according to Shlomo Avineri, professor of political science at Hebrew University who is close to the opposition Labor Alignment in Israel.

He said Tehiya eventually might have joined the coalition anyway to improve its chances of political survival. But the invasion of Lebanon, which provoked calls here for unity during the "national emergency," provided the three Tehiya members with a "legitimate" reason to join forces with Begin now rather than later, Avineri said.

Last fall Begin increasingly appeared to be politically vulnerable, barely surviving two Knesset votes of confidence. But just before the invasion of Lebanon another small opposition party dissolved itself, strengthening the government somewhat.