A 55-pound bomb attached to a mortar shell exploded in a crowded neighborhood in west Beirut yesterday, killing three persons in the second bombing directed at a civilian target in three days.

Police said the blast outside the Bank of Beirut and Arab Countries, minutes after its closing, killed three persons and wounded 27 others, wrecking cars and damaging nearby buildings, United Press International reported.

"This is the first act in a warning addressed to Druze leader Walid Jumblatt to get his Progressive Socialist Party militiamen out of Beirut," a man who said he represented the previously unknown Youths of Free Beirut group told the Christian Voice of Lebanon radio. A Druze spokesman dismissed the warning as a "hoax to fuel dissension between Moslem factions of west Beirut."

Another anonymous caller to a foreign news agency said that Islamic Jihad, Beirut Command, was holding the Rev. Lawrence Jenco of Catholic Relief Services, who was seized by gunmen on Tuesday, and four other Americans kidnaped earlier. The man said the five would be freed if all U.S. citizens left Lebanon.

In Jerusalem, Israeli Defense Minister Yitzhak Rabin said he had submitted to the Cabinet a plan for phased withdrawal of troops from southern Lebanon. But Foreign Minister Yitzhak Shamir expressed opposition to Rabin's plan.