Airlines -- looking almost as desperate as retailers to attract customers -- are throwing more incentives at their frequent fliers.

American, Northwest and United Airlines yesterday joined Delta Airlines in offering free round trips to frequent fliers who make a certain number of flights in the next few months.

Enticing frequent fliers to step up their frequency is just the latest of the airline gimmicks aimed at boosting lagging domestic traffic.

Last month's gimmick was a buy- one, get-one-free companion ticket promotion that analysts predicted would probably end up costing the airlines money.

At the heart of the promotional fervor seizing the airlines is the sorry state of the industry. Devastated by higher jet fuel prices and weakness in the market for air travel, the nation's major carriers are stepping up efforts to retain market share in a puny market.

In the case of the current promotion, airlines are betting that the frequent fliers, many of whom travel extensively and at some of the highest fares at someone else's expense, will channel their business to the carrier that offers the promotion to win a ticket for a personal trip.

American, Northwest and United said yesterday that frequent fliers can earn a free round trip by flying as few as three round trips or eight flight segments. A flight segment is a one-way, nonstop flight. Northwest boasted that it is the only airline to include free travel to Europe and Hawaii.

That tops Delta's offer earlier this week of a free trip for four round trips without connections or eight segments.

In the American and United versions of the promotion, five paid round trips or 14 segments garners two free round trips, and seven paid round trips or 20 segments is good for three freebies.

The qualifying flights must occur between this Saturday and March 15.

The free tickets must be used between May 15 and Dec. 31, 1991.

Several other airlines, including Continental, Pan American, Trans World and Eastern, said they are considering matching the promotions.

Eastern actually may have prompted Delta's promotion. A week before Delta announced its program, Eastern -- Delta's chief competition in its Atlanta airport stronghold -- said that it would offer a $91 round trip ticket anywhere in the United States to fliers who paid the corporate rate.

TWA said that it has been offering fliers who fly two round trips across the Atlantic between Oct. 15, 1990, and Jan. 31, 1991, a free round trip coach ticket to anywhere TWA flies except Tel Aviv and Cairo.