Three airlines announced reduced fares yesterday from Washington or New York to Florida in a rapidly escalating air-fare war.

As airlines scrambled to offset a seasonal slump further aggravated by the air traffic controllers strike, Eastern Airlines said it will offer the lowest unrestricted fare from the Washington area to Miami in more than 10 years. Eastern said it will charge $79 for one-way coach fare ($99 on weekends) on its daily Flight 195, departing National Airport at 8:40 a.m. and arriving in Miami at 10:55 a.m. The fare is available for a return trip on Eastern Flight 190, departing Miami at 10:55 a.m. and arriving at Washington National at 1:03 p.m. The new fares begin today.

Pan American World Airways said it will drop fares as of Sept. 14 to $75 one-way coach class between New York and Washington to Miami during midweek and $95 on Friday, Saturday and Sunday. The fare cut was a further reduction from the $79 weekday and $99 weekday fares it announced last week.

Both the Eastern and Pan American fares will be available till mid-November when holiday travel and cold weather in the Northeast normally begin to alleviate the airlines' problems.

Also as of Sept. 14, Air Florida will offer a $69 one-way coach fare between LaGuardia Airport and Newark and Miami, Ft. Lauderdale and West Palm Beach Monday through Thursday -- the lowest of the snowbird fares. On Friday, Saturday and Sunday the fare will be $89.

"These fares are getting down to where they've never been before," a spokesman for Pan Am said yesterday. September fare-cutting is traditional with airlines, which also tend to raise their fares when temperatures begin to fall sharply. But this year's cuts come with fewer restrictions and are to somewhat lower levels.

Existing fares from New York to Miami are $296 for a one-way, first-class ticket; $218 to $245 for a one-way coach ticket; and $358 to $416 for a round-trip "super saver" ticket which must be purchased in advance and comes with other restrictions.

Current fares for a flight from Washington to Miami are $242 to $324 for a one-way first-class ticket, $196 to $232 for a one-way coach ticket and $340 to $358 for a round-trip "super saver" ticket.

In other fare reductions, Eastern said it will cut its fares between Miami and San Juan from $119 to $79 for one-way trips on weekdays and from $140 to $99 weekends. Reservations and ticket purchases for these flights must be made a week in advance of travel.

In the New York-Los Angeles-San Francisco markets, Pan Am said it will add a $190 standby one-way fare, which it said would be the lowest of any major carrier on the routes. The airline also said it will reduce its one-way unrestricted normal economy fare to $219.

Pan Am also said it will reduce its normal economy fare between Houston and Los Angeles to $95. First-class fare will be reduced to $199. One-way normal economy tickets for travel between Miami and San Francisco will cost $174. First-class fare will be $294.

One-way economy fares between Miami and Phoenix are to be cut to $150, with first-class fare reduced to $294.