Federal Savings and Loan Insurance Corp. is expected to use the new S&L it created this week to borrow money against the $3.1 billion in real estate and other assets the insurance fund has taken over from failed thrift institutions, according to sources in the industry.

The new organization, called the Federal Asset Disposition Association, would funnel some $2.6 billion to FSLIC and use the remaining $500 million for operations and debt service while searching for ways to dispose of the assets that would bring in the most possible cash, according to sources at the convention of the U.S. League of Savings Institutions here this week.

Although final decisions on how the new organization will work have not been made, the plan for borrowing funds -- probably from the Federal Home Loan banks -- appears to be the most likely scenario.

The idea for the new organization was conceived by a group of thrift industry leaders, and its petition for a federal charter was approved by the Federal Home Loan Bank Board this week. The bank board granted the approval as a first step toward strengthening the hard-hit FSLIC, said Edwin J. Gray, board chairman.

The high-flyers of the recently deregulated thrift industry have invested in risky real estate projects in an overbuilt market as well as in a wide variety of other businesses. Some of the more unusual investments, according Norman Raiden, general counsel of the bank board, include restaurant chains, a mushroom farm, a tire shredding company, a ski lodge, a heavy equipment business and some "strange joint ventures."

The FSLIC, as a result of widespread thrift failures, now finds itself the unhappy owner of many such properties.

The insurance fund's shaky condition is a major concern to the bank board, the thrift industry and Congress. Several solutions have been proposed, but the new association is the first to be put in motion. Although favored by the industry, formation of such an organization reportedly was viewed without enthusiasm by the Reagan administration and bank board chairman Gray.

Where money for the organization will come from is the biggest unanswered question.

"If the corporation buys assets and holds them for itself, then it would be quite a large corporation," Raiden said. "It would need sources of borrowing and it would need some equity investment. But . . . if the new association became a management company working under contract for the FSLIC then sources of funding would be different," Raiden said.

If the new association borrows money to buy the FSLIC's assets, the loans probably would come from the federal bank system, said Dennis Jacobe, senior vice president for research U.S. League of Savings Institutions, who is familiar with the new S&L.

Whether the loans come from federal banking system or public institutions, they would be fully collateralized with the property bought from the FSLIC, he said. If necessary, the FSLIC could guarantee the loans, he said. But he added that this is one of the many questions not answered.

Organizers of the new association proposed that the home loan banks provide some of the funding by buying $1 billion worth of stock in the new S&L.

"We have not concluded that the banks have the legal" authority to buy the stock, Raiden said. "I have asked that the banks each obtain opinions from their counsels about whether they believe it's legally justified."

FSLIC losses have already been recorded, and transfer of the loans and property to the new association will not mean an additional loss for the insurance firm, according to Jacobe. The $3.1 billion figure assigned to the assets that will be transferred represents the market value of the property, he said. Reserves to cover the difference between market value of the assets and the original loans values have already been set aside by the FSLIC, he added.

The bank board chose to set up an organization to dispose of the assets over a longer period rather than hire liquidators to dispose of the property for immediate cash because it believes the assets would grow in value during the time that the new association is looking for the most profitable ways to dispose of them, Jacobe said.

The insurance corporation bought a thousand shares of the new association for $1 million to provide start-up capital. The association is expected to service the debt either from borrowed money or from cash flow from the sale of assets.

As the association sells off the properties it initially takes from the insurance corporation, it will take newly acquired loans and property from FSLIC.

In addition to the property FSLIC is already holding, another $15 billion worth of assets is in the bank board's management consignment program for failed S&Ls. These institutions are being managed by other, successful savings and loans until their bad assets can be sold. Once the insurance fund takes out the bad assets, the institutions themselves will be sold.