IT IS BARELY three months since President Reagan launched his Mideast plan, and already there are complaints that his initiative is dead. Some say this by way of urging the United States to put pressure on the consensus villain, Israel, and others say it out of genuine despair. Time, they agree, pointing to the concrete Israel is pouring in the West Bank, is not on the president's side.

Time is short. But arbitrary deadlines have no merit. Surely more than three months is required for an initiative addressing a dispute that has festered for decades. The administration does not appear to us to have lost its September resolve. It struggles on, chiefly these days in Lebanon, which is now generally accepted as the place where the first fruits of its seriousness must be shown.

How is it going? The Israelis are scaling down their conditions for withdrawal: they have stopped insisting on a peace treaty and ministerial-level talks. But they are still trying to use their occupation to impose a relationship that Lebanon's government rejects. The Israelis can reasonably expect to make effective but discreet arrangements for border security. To ask for more now is overreaching.

What would move the Israelis out? A parallel exit of Syrian and the remaining PLO troops would be a big help. Each member of this trio is in a position to ensure, by dragging, that the others will drag, too. Those Arabs who are so ready to call for Israel to withdraw can best make their own appeals come true by applying their influence where they may have influence, on the Syrians and PLO.

It would also help if Washington were readier to acknowledge the need for more extensive international forces, including American forces, to keep the peace in the parts of Lebanon to be evacuated. The Lebanese government has its own air of unreality in this regard. The sad truth is that the Lebanese remain deeply distrustful and heavily armed, and outsiders will have to stay while the government practices asserting its authority. There's no other way.

Cut American aid to Israel? That's far too severe a step to take while the bargaining in the Beirut bazaar goes on. It would require a decision that at this moment seems unjustified. To be sure, the Israelis do not make restraint easy. As we read it, when Mr. Reagan promised not to use aid as a lever against Israel, implicitly the Israelis promised not to use aid as a lever against him, as they have been doing by lobbying in Congress for $475 million more than he requested for them. They will not get that extra aid, since the bill containing it will be superseded by a continuing resolution. They should not get it. They should get out of Lebanon.