THREE ISRAELIS have been sentenced to life imprisonment and 12 others to terms ranging up to 10 years for acts of terror against West Bank Arabs. The 15 had been convicted of a range of offenses including murder by machine gun, the attempted assassination of three Palestinian mayors and a hideous plot to blow up one of Islam's holiest shrines, the Dome of the Rock in Jerusalem.

Israel had, fairly, reaped much credit for finally applying the law to the terror network that the authorities had allowed to develop among Jewish settlers on the West Bank. The case of the 15, however, is now moving back from the courts toward the political arena. Moves are afoot among the criminals' many and fervent supporters to gain clemency by legislative action and to press the head of state, President Chaim Herzog, for pardons.

In the battle against Arab terrorism Israel has been generally successful, having managed in circumstances difficult beyond most Americans' imagining to live a normal national life, and to do so while still preserving Israel as a free society. In recent years, however, Israelis have been tormented by the spectacle of a strain of Jewish terrorism too.

Unfortunately, the deputy prime minister, Yitzhak Shamir, spoke for many when he described the 15 convicts as "excellent people who made a mistake." The way to ensure that there will be no more Jewish underground, he said, was to free the 15. How can Israel imprison 15 of its own, others add, when it has just yielded up 1,150 convicted Arab terrorists in a trade for three Israeli POWs?

The Israelis will have to make their own choice. On it, they know, rests a fateful question of the definition of their society. On it also rests a question of the reputation of their society.

Israel's claim to a special kinship with the West arises from the fact that, unlike any Arab nation, it shares the democracies' professed reliance on the sanctity of the law. This is the basis on which Israel comes to the United States and other countries and asks for a partnership against all forms of the international crime of terrorism.

This claim puts upon Israel the painful requirement of showing that its aversion to terrorism extends to Jewish conduct as well as Arab conduct. The judicial system has worked its will on the 15 terrorists, and now it is the political system's turn.