Three ethnic, nationalist groups -- the most visible of the terrorist groups operating in this country -- are "fighting their war on U.S. soil," in the words of an FBI terrorism expert.

The three are a Puerto Rican group known as FALN, a group calling itself Croatian Freedom Fighters and an anti-Castro exile group known as Omega 7.

Of 52 domestic terrorist incidents in 1979, 30 were attributed to the FALN, FBI statistics show. Omega 7 was believed responsible for 10 others. f

"These three give us the most problems," FBI Director William H. Webster told reporters at a luncheon last week.

In many instances, the terrorists are waging a war for words, aiming their actions at publicity, as did the brief FALN seizures a month ago of the Carter-Mondale headquarters in Chicago and the George Bush campaign office in New York.

But federal officials are concerned that the battle could escalate, with bloody consequences.

Webster noted recent killings by other Puerto Rican independence groups in San Juan the first instance of political assassinations in the United States, its commonwealths or territories since the 1976 murder of former Chilean ambassador Orlando Letelier in Washington.

The FBI director said recent events "suggest the development of a serious capacity inside this country" for similar assassinations. When police in Evanstan, Ill., arrested 11 alleged FALN members this month, the resultant searches turned up caches of arms and explosives, maps of national political convention sites and files on 50 corporate executives.

The April 4 roundup "probably represents the hard core of the FALN," Webster said in the meeting last week.

But while the arrests focused attention on the Puerto Rican groups, the FBI also has become increasingly concerned with the violent activity of the anti-Castro Cubans. There is an irony in this federal attention, because many of the Cubans got their explosives and weaponry training from the Central Intelligence Agency in the early 1960s.

The militant U.S. policy against Castro has changed since then, but the Cuban exiles have not. "We put the fathers in jail and the sons take their place," an FBIterrorism specialist said. "They have a cause."

Typically, the terrorist groups use violence to force attention to their causes, though it is FBI policy not to yield to terrorist demands. In mid-March the Croatian Freedom Fighters claimed responsibility for the bombing of a bank in New York City, and demanded immediate publicity for their call for an independent Croatia and a stop to U.S. financial and military aid to Yugoslavia.

But groups representing separate Serbian and Croatian terrorist factions have in the past sought to advance their cause through assassination and random bombings, a matter of concern to FBI experts.

Present FBI guidelines for use of informants and intrusive surveillance techniques are adequate to deal with terrorist groups now operating in the United States, in Webster's opinion. About a dozen groups and 50 individuals are now targets of such domestic security investigations, he said.

Roger Young, the FBI's chief spokesman, who a few years ago coordinated the search for Carlos Torres, one of the alleged FALN members arrested in Evanston, said the main problem tracking such groups is that they are small and, with the help of sympathizers, blend into the communities where they live.

Young noted, for instance, that the FBI was chasing FALN -- for Armed Forces of National Liberation -- for some time after a bomb killed four persons at historic Fraunces Tavern near New York's Wall Street in 1975 without knowing who the suspects were. Torres was first identified after a Chicago burglar was arrested for selling dynamite he'd stolen from Torres' apartment in 1976.

The FALN arrests this month also were the result of a lucky break -- police were tipped by a suspicious neighbor. "The strongest weapon we have going for us today in fighting terrorism is the alertness of the local populace and police," Young said.

He also emphasized that in its efforts to stamp out terrorism the FBI isn't trying to muzzle political dissent. "We're interested in solving bombings and preventing murders," he said. "We're not trying to destroy the Puerto Rican independence movement or any other political cause. We only go after someone when they turn to violence."