The U.S. Department of Transportation has given Metro three years to come up with a guaranteed source of local tax funds to pay back $1 billion of subway construction debts and will cut off further federal aid in 1982 if Metro fails to do so, the transit authority announced yesterday.

The Metro board, concerned as always about money, also agreed to explore a revamping of the transit system's bewildering fare structure -- a step that could lead to a single flat fare to ride a bus or train anywhere in the region.

In another step that should make train riding easier for fringe-parking motorists, the board approved three experimental actions starting Nov. 1.

The often balky automated parking lot gates at the Cheverly station will be replaced by cashiers. Monthly parking permits costing $25 each will be sold for one 323-space lot at New Carrollton. Parking meters will be installed in some other spaces at New Carrollton, as well as at the Rhode Island Avenue and Minnesota Avenue stations in Northeast Washington.

A proposed agreement setting the 1982 deadline for imposing local taxes throughout the region to repay the Metro debt was announced by William A. Boleyn, Metro's chief financial officer, who called the entire package "a major breakthrough."

It calls for the federal government to provide $2 billion toward the repayment of the Metro debt if the local governments in Maryland, Virginia and the District of Columbia find a guaranteed way to produce half that amount over a 33-year period.

The debt is in the form of revenue bonds that were sold by Metro between 1972 and 1975. They were to have been paid off from profits anticipated from fare collections -- but inflation and the Metro board's policy of holding the line against steep fare increases is producing huge losses instead of a profit.

The bonds have a face value of $997 million. The cost of interest payments will drive the cost of retiring the bonds up to an ultimate $3 billion.

Brock Adams, until recently the secretary of transportation, had insisted for a time that federal assistance in paying off the bonds would depend upon the adoption of some form of regionally uniform tax by area governments.

Because any such tax would require identical actions by the two state legislatures and the D.C. City Council or Congress, the odds against adopting such a tax are enormous. So the Department of Transportation, in negotiating the proposed agreement, is insisting upon each jurisdiction adopting "a dedicated source of revenues."

That means each jurisdiction could choose its own revenue source, which would be acceptable just as long as it was guaranteed to produce enough money.

Attempts for the past two years to allow Northern Virginia localities to impose a special one-cent sales tax for Metro were rejected by that state's General Assembly. Maryland officials also have resisted special taxation.

The proposed agreement between Metro and the Department of Transportation must be reviewed but not formally signed by officials of the eight local governments that are partners in the Metro program.

Once it is signed, the eight governments would be required to make interest payments and also to contribute to a special new fund -- called a "sinking fund" -- that will pay off the face value of the bonds when they mature between the years 2012 and 2014.

For most years, the total cost of the region's taxpayers would be $28 million. There is a one-time payment of $171 million that would fall due in 1993, to repay the U.S. Treasury for money now being advanced to Metro to pay interest on the bonds.

Giving muscle to the agreement, the federal agency would have power to stop granting further funds to Metro -- for construction or for any other purpose -- if the money sources are not fully nailed down by Aug. 15, 1982.

That would leave Metro without money to complete some still unbuilt routes on its planned 101-mile rail system, including those to Greenbelt, Rosecroft and Franconia-Springfield.

The board's agreement to explore changes in Metro's complex bus and rail fare structure was informal. It came as the board decided to cancel a closed meeting that had been called for later yesterday by board chairman Jerry A. Moore Jr. to discuss the future of Metro's trouble-planned Farecard system, which vends tickets and collects fares.

For that session, Richard S. Page, Metro general manager, prepared a report that dealt with fares as well as Farecards. Although the report was not released publicly, sources said it contained preliminary suggestions for possible changes in fares, including a single flat fare or the use of a few large fare zones for both buses and trains.

Moore's meeting was canceled by agreement of the Metro directors after reporters questioned the legality of a closed session discussing public policy matters.

On other matters:

A group of 36 workers in Metro's track and structures department submitted a report contending that they must work under conditions and practices that are dangerous. Page promised a thorough investigation.

An investigating team reported that a collision between a diesel-powered work train and a train of regular Metro electric cars shuttling employes at 12:15 a.m. May 14 at the L'Enfant Plaza station was caused by a dispatcher's error. Seven employes were injured in the accident, none seriously.