West German police today dismissed a claim by the Arab Revolutionary Organization to have planted the bomb at Frankfurt's Rhein-Main International Airport Wednesday that killed three persons and hospitalized 27 others, a police spokesman said.

The previously unknown group assumed responsibility for the explosion in a message passed to a newspaper in Beirut on Thursday night. It said the attack was carried out because West German intelligence agents were working with their U.S. and Israeli counterparts to recruit young Arabs in West Germany to assassinate leading figures among "Arab fighting organizations" in Lebanon.

A 92-member commission investigating the bombing concluded today the claim was a hoax because the group had never been heard of before and its statement lacked sufficient details.

The powerful blast occurred in a departure lounge beyond the airport security zone, near the check-in counters for international flights. The explosion tore a crater three feet deep in the concrete floor, littering the area with ripped luggage and broken glass.

Two Australian children and a Portuguese man were killed. Four of those injured are still listed in critical condition.

Police officials said they also rejected half a dozen telephone calls assuming responsibility for the bomb, including two claims by people purporting to be associated with the extreme-left Red Army Faction.

Heribert Hellenbroich, head of West Germany's counterintelligence activities, said today he did not believe the Frankfurt attack was the work of radical left- or right-wing German terrorists. He and other senior investigators contended that the blast did not fit in with the kind of tactics used in the past by such groups.

The explosives were so potent that all remnants of the bomb were apparently "atomized" in the blast, making the effort to trace its origins difficult, a police spokesman said.

Frankfurt investigators say the best lead so far has come from a Portuguese passenger who saw a man running from the scene after the explosion and driving off in a blue Mercedes.

Reached by West German police in Lisbon, the passenger described the man as about 5 foot 7, about 30 years old and slim with dark hair, authorities said.

Police examiners said they have determined that the bomb weighed 10 to 20 pounds and was hidden in a gray suitcase next to a wastebasket.