The deaths of three Israeli soldiers in less than 24 hours this week was a stark reminder to Israelis of the cost of a continued military presence in Lebanon and is likely to increase pressure on the government of Prime Minister Menachem Begin to bring the troop withdrawal negotiations with Lebanon to a conclusion.

The soldiers, who included two officers from an elite combat unit that played a key role in the early days of the Lebanon war, were killed in two separate incidents in southern Lebanon Wednesday night and yesterday morning. Their deaths brought the number of Israeli soldiers killed in Lebanon since September, when the Palestinian guerrillas who were the target of the Israeli invasion withdrew from Beirut, to more than 130.

Last month, three Israeli soldiers were killed and 19 others were wounded as a result of clashes in Lebanon. The toll so far this month is six killed and 11 wounded. With the coming of warmer, dry weather to Lebanon, Israeli officials concede that the number of clashes with guerrillas is likely to increase.

The casualty toll, coupled with conflicting reports about the troop withdrawal negotiations that have been going on since December, has stirred restiveness here after a winter in which the Begin government was able to pursue its objectives in the negotiations with Lebanon free of serious internal pressure for a rapid Israeli withdrawal.

Leaders of the opposition Labor party, who have been largely silent on the issue, have begun to express impatience with the slow pace of the talks and to question some of the goals the Begin government has set for itself in the negotiations.

Moreover, there are renewed rumblings within the government itself for a unilateral Israeli withdrawal to south of the Awali River, where the Army presumably would be in less danger of attack by guerrillas infiltrating from behind Syrian lines.

The notion of such a unilateral withdrawal--which would throw on the Lebanese the responsibility for policing the areas evacuated by the Israelis--has surfaced periodically during the negotiations but has never been seriously considered by the government. Nevertheless, the idea reportedly is to be brought up by some government ministers, including Minister Without Portfolio Mordechai Ben-Porat, at the next Cabinet meeting on Sunday.

In apparent recognition of growing dissatisfaction with the situation, Defense Minister Moshe Arens, Foreign Minister Yitzhak Shamir and members of the Israeli negotiating team met at the Defense Ministry in Tel Aviv today to review the state of the negotiations after what was described here as yesterday's "frustrating" and "disappointing" negotiating session with the Lebanese.

After the meeting, Shamir gave a speech in Tel Aviv in which he appealed for patience despite the casualty toll. Conceding that the government is coming under increased questioning on the Lebanon issue, Shamir said Israel should not allow "the national tragedy of fallen soldiers" to lead it into "defeat and surrender."

And if, after agreement is reached with the Lebanese, the Syrian Army refuses to leave Lebanon, Shamir said Israel will "dig in" and "find ways to avoid casualties."

The foreign minister stressed in the same speech that Israel is eager to leave Lebanon, but only if it achieves adequate "security arrangements" in the south, and that it has no desire or intention of becoming involved in a military clash with Syria.

The frustration over the most recent casualties appears to have been compounded by a tide of rising expectations that has been encouraged by Shamir and other officials. In recent weeks, the Israelis have spoken optimistically about agreement with the Lebanese being within reach. Shamir recently predicted that a troop withdrawal agreement could be concluded in two weeks.

But the Israeli press was filled today with accounts of lack of progress at yesterday's negotiating session in the Beirut suburb of Khaldah.

Briefing foreign correspondents today, an Israeli official complained that the Lebanese had suddenly balked at putting into formal written form certain understandings that the Israelis thought had already been reached. He said these included the understanding that the militia units commanded by Saad Haddad, a former Lebanese Army officer allied with Israel, would be integrated into the Lebanese Army and play a key role in patrolling southern Lebanon after the Israelis withdraw.

But the official said it will not be clear whether the negotiations are headed into a new stalemate until the talks resume next week.

These developments have been taking place amid a noted absence of public leadership by Begin, who has had virtually nothing to say about them and is rarely seen in public. The prime minister did not attend any of the Independence Day ceremonies held here last Monday, and he canceled a scheduled appearance later in the week before a United Jewish Appeal group from the United States.