THREE OTHER Soviets in Lebanon were being held prisoner even as the report came that a Soviet diplomat had been executed by an Islamic fundamentalist group fighting in Tripoli against Lebanese elements backed by Syria and, through Syria, the Soviet Union. Few people, we surmise, are going to be able to deplore the murder without wondering whether it may not be therapeutic for the Kremlin to get a taste of the medicine that it has long been prescribing -- by its various forms of encouragment for international terrorism -- for others. But this in itself is not an adequate, let alone attractive reaction.

It appears to be the first time that Soviet citizens have been taken hostage and harmed in the charnel house of Lebanon. As a result, there is a considerable curiosity about what the Kremlin may do. One theory has been that terrorists hesitated to hit Soviets because they feared an immediate and crushing response. Now we will see whether Moscow moves to protect, remove or avenge its people, and whether it slackens or keeps up or even increases its support for its Lebanese friends in the field. These are important questions bearing on the whole Soviet role in the Middle East.

It is painful but necessary to note that in somewhat analogous circumstances of threat to its Lebanese presence, the United States abandoned its role as a would-be patron of the country's integrity and unity. In a largely unnoted sequel, the Soviet Union then deliberately moved into some part of that same role -- principally by standing behind Syria, which regards itself as the single legitimate overseer of Lebanon.

Syria has been conducting a hard policy of enforcing order -- of a kind -- in Tripoli and of otherwise trying to assert control in Lebanon. The United States is not pleased with the means, but in the absence of any other feasible way to stop the bloodletting and disintegration, Washington has quietly endorsed the Syrian policy. It has done so notwithstanding Syria's own readiness to sponsor and condone terrorism, including, it is believed, terrorism directed at Americans. In so doing, Washington has acquiesced in the Soviet supporting role.

This leaves the United States in a strange position with respect to the latest hostage-taking. Even as it takes a consistent position against terrorism, this country cannot fail to hope against hope that Moscow will review its cynical support of terrorism. At the same time, the United States finds itself forced to acknowledge the job that Moscow is performing in Lebanon -- one that Washington largely yielded.