After negotiating all night as strike and loan-default deadlines loomed, Eastern Airlines and its flight attendants union reached a tentative three-year agreement yesterday, which they said would avert a walkout and also may solve the carrier's financial dilemma.

Robert Callahan, president of the local Transport Workers Union, yesterday called the agreement "the best possible . . . given the absolute insanity of the last year."

Eastern's flight attendants were set to strike at midnight last night. The agreement reportedly calls for 20 percent wage cuts and an increase in work hours, which will replace similar wage cuts and work increases that the company temporarily instituted last month.

On Tuesday, Eastern and its pilots signed a three-year contract in which the carrier's 4,200 pilots accepted 20 percent wage cuts and work-rule concessions.

Meanwhile, Eastern has lowered fares to more than 100 cities during the first three weeks in March starting today because bookings fell drastically because of the strike threat. USAir also announced yesterday that it would shave between $20 and $40 off supersaver fares for domestic flights between April 1 and May 22, except for flights to and from Florida.

In another development, Eastern and Texas Air Corp., in a joint filing with the Department of Transportation, have asked for quick approval of Texas Air's proposed acquisition of Eastern.