Eastern Airlines and representatives of its pilots' union hammered out proposed pay concessions yesterday intended to head off a strike Wednesday at the nation's third-largest airline, union sources said.

The negotiators' proposal was being considered late last night by the executive council of the Air Line Pilots Association, which has the authority to approve any new contract. Terms of the proposal were not immediately available.

Pressure on the pilots was intensified by an offer from their nemesis, Frank Lorenzo of Texas Air Corp., to buy Eastern for an undisclosed amount. While the labor negotiators wrestled with contract terms, Eastern's directors were poring over Lorenzo's offer in Eastern's Miami headquarters.

There was no word on the Eastern board's response to the purchase offer by Lorenzo, whose corporation owns Continental Airlines and New York Air.

Eastern Chairman Frank Borman has told the pilots that a permanent 20 percent pay cut and new work rules are essential if the airline is to remain in operation. The pilots earlier agreed to pay cuts, which would save Eastern $300 million over three years, but demanded a commitment to restore the lost pay if the airline recovers financially.

Union sources said the proposal made last night by the negotiators includes wage concessions, but would give no details.

The pilots have voted to strike at 12:01 a.m. Wednesday if there is no new contract, and management has said it will shut down if struck.

But pressure on the pilots to settle was heightened, some sources said, by the intervention of Lorenzo, who has been the bane of the airline unions since he took Continental into a Chapter 11 bankruptcy proceeding and broke its labor contracts in 1983.

Borman did not identify the bidder when he revealed the purchase proposal to union representatives Saturday and company spokesmen declined comment yesterday, but other sources confirmed the proposal.

"The proposal is real. The proposed buyer is Lorenzo," said Charles G. Bryan of the International Association of Machinists and Aerospace Workers, the leader of Eastern's mechanics and a member of Eastern's board of directors.

Lorenzo is in the process of bringing Continental out of its bankruptcy reorganization with a plan to repay creditors and has several hundred million dollars in available capital to help finance an acquisition of Eastern, sources said.

For years, Eastern has been struggling to defend its territory against lower-cost competitors such as Continental and People Express, whose pilots and crews are nonunion. Eastern lost $355 million in the last five years, but managed a $6.3 million profit in 1985, thanks to strong performance from February through August.

Since then, the airline's financial condition has taken a sharp turn downward, with Eastern citing fare wars and intensifying competition, and the union blaming the reversal on airline mismanagement.

Eastern is $2.5 billion in debt and its lenders have told the airline it must obtain substantial, permanent cost-cutting agreements with its unions by Friday, or be in technical default on its loans. At that point, the lenders could demand accelerated repayment, or begin claiming Eastern's aircraft or other assets.

Two days before that is the pilots' strike deadline. Eastern's 7,200 flight attendants have set a March 1 strike deadline.

With the lenders' deadline and the strike deadlines drawing close, Eastern was experiencing a "serious" drop in advance reservations, an airline spokesman said last week.

Eastern shuttles, between Washington and New York and Boston, and its new South American routes are profitable, as are some budget flights. But elsewhere its system is in serious trouble.

Lorenzo's New York Air recently mounted an advertising offensive against the Eastern shuttle, and Continental has damaged Eastern on other routes.