Only three days after Israel began its withdrawal from southern Lebanon, it has become brutally clear to officials and the Israeli public that the process of leaving Lebanon is going to be a bloody, costly affair.

Sgt. Shlomo Avrumov, Maj. Shaul Zehavi and Col. Avraham Hido were the first to die after the withdrawal had begun.

Avrumov, 23, was killed by a blast from a roadside explosive around noon on Sunday, less than 24 hours after the Israelis had completed pulling out of Sidon and the surrounding area in the first stage of the planned three-stage withdrawal.

Zehavi, 27, was killed yesterday, also by a roadside explosive in the same area east of Tyre where Avrumov died.

Also yesterday, Hido, 41, the senior liaison officer with the Israeli-backed South Lebanon Army, was killed when his unit was attacked in the eastern Lebanon countryside by small-arms fire and a rocket-propelled grenade.

Their deaths, plus the wounding of an Israeli soldier in eastern Lebanon today, brought the totals for the first three days after the initial pullback to three killed and five wounded. According to Israeli Army figures, since Jan. 14, the day the Israeli Cabinet approved the withdrawal plan and announced its intention to leave Lebanon, 15 Israeli soldiers have died and 104 have been wounded as a result of an increasingly bold Lebanese resistance.

The total Israeli casualty toll since the outset of the June 1982 invasion is 621 killed and 3,711 wounded.

Senior Israeli officials have been acknowledging for weeks that the withdrawal would be painful and would be accompanied by more attacks on Israeli forces by Lebanese Shiite Moslem militias.

At the moment, they have shown no public inclination to change their withdrawal plans. But as the abstract warnings are transformed into the reality of more dead and wounded soldiers, the national-unity government here is likely to be caught in a whiplash between critics on the left, demanding an accelerated withdrawal schedule, and those on the right, who warn that each stage of the pullback brings the threat of terrorist attack nearer to Israel's civilian communities along the northern border.

Even before the latest casualties, questions were being asked here about why, if Israel intends to withdraw to its own border, it should drag out the process, pulling back in three distinct stages with a pause of several months between stages.

In a response to such criticism, Defense Minister Yitzhak Rabin said in a newspaper interview today that he remained opposed to a "hasty withdrawal."

"The matter seems simple on the face of it," Rabin said, "but the redeployment requires time so that we can build an infrastructure on the international border in order to ensure peace for Galilee."

The withdrawal is now tentatively set to be completed by the summer. But the Cabinet must approve the timing of each stage, and Rabin in the interview today with the independent newspaper Haaretz refused to speculate on the final timetable.

"I don't want to get into dates, but leaving the eastern sector stage two in the plan will be harder than leaving the western one the just completed first stage and the final stage will be the most complex," he said.

Military officials here acknowledge that Israel's new deployment in southern Lebanon makes its forces more vulnerable than they were just a week ago.

It lacks the permanent fortifications and bases the Israelis built and then dismantled along the Awwali River, and the area is dotted with Moslem Shiite villages from which, according to Rabin, 90 percent of the attacks on Israeli soldiers have come.

"It's not a line at all. It's a temporary deployment," said Lt. Gen. Moshe Levy, the Israeli Army chief of staff, on Saturday as he oversaw the pullback from the Sidon area.

The response of Rabin and the military to the continuing attacks has been to promise an increasingly aggressive defense of the Army while it is in Lebanon, and of the Israel's northern border after the troops have been withdrawn.

The Israelis have already begun that process, with two air attacks against Palestinian bases in Lebanon last week and a series of searches of Lebanese Shiite villages where some houses were destroyed and dozens of people were arrested.

Moreover, Rabin said in a speech today, if Israel's new enemies among the southern Lebanese Shiites carry their assaults to the border -- as the defense minister and senior Army officers have publicly predicted they will -- Israel will act to prevent this "even if it will require entering back into Lebanon temorarily, bombing the area, shelling it."

A military official said that military planners hope that, now that the Sidon area has been abandoned, Israeli forces will be more concentrated and will exercise more effective control over the territory they still occupy.