Apparently rebuffed in an effort to gain control of District of Columbia National Bank, 31-year-old investor Jeffrey N. Cohen had decided to sell his stock in that bank and now is trying to buy controlling interest in two smaller District Banks.

Cohen said yesterday he has agreed to make a public offer to pay $7 million to buy all the stock of Bank of Columbia, a $50 million bank with three offices.

Cohen said he also has signed an option to buy for an undisclosed sum "more than 50 percent" of the stock of Hemisphere National Bank, second-smallest in the District with deposits of $10.9 million.

At the same time, Cohen has given another local investor an option to buy his 29 percent interest in D.C. National for a reported $3 million.

None of the three deals has been completed, and any one or all of them could fall through, Cohen stressed in an interview yesterday.

Part of the money to buy Hemisphere and Columbia could come from the sale of his stock in D.C. National, Cohen said, adding that he "plans to put together a group of investors" to provide the rest of the capital.

Both the sale of Cohen's D.C. National stock and the purchase of interest in Hemisphere and Columbia require the approval of federal banking regulators, who generally frown on speculative investment in banks.

Cohen's three-cornered bank-buying plan is the latest in a series of recent shifts in control of Washington banks that have pushed the price of local bank shares to record levels.

The reason for the great interest in Washington banks is the expectation that Congress soon will change the law that prohibits interstate banking and allow banks to conduct business in more than one state, said Charles Brodhead, who follows bank stocks for the brokerage firm of Johnston, Lemon & Co.

Most bankers believe the Washington area will be the first in which interstate banking will be permitted, because local financial institutions already deal with customers in three different jurisdictions.

If that happens, Brodhead explained, a Washington bank charter will become an extremely valuable investment, since the easiest way for a big Baltimore or New York bank to break into the District market will be to buy a small local bank.

Because of the speculation on the future potential of Washington banks, the selling price of most local banks' stocks has increased dramatically in the last couple of years, he added. Stocks that were selling for 50 percent or 60 percent of the book value of the bank's assets now are trading for 150 percent to 200 percent of book value when control of the bank is at stake.

Cohen has agreed to offer shareholders of Bank of Columbia $16 a share for stock whose book value is about $9.50. That offer has been accepted tentatively by holders of about 160,000 of Columbia's 440,000 shares, Columbia Chairman Donald E. Wolpe disclosed in a letter sent to stockholders last week.

The letter did not identify the investors who have agreed to accept Cohen's offer, but Wolpe confirmed that he is one of them. Also reportedly willing to sell is another director and major stockholder of Bank of Columbia, attorney Douglas Drysdale of the firm of Caplin and Drysdale.

Bank of Columbia's board of directors has given Cohen until Dec. 20 to review the bank's books and decide whether to make a formal offer for the stock. Under terms of an agreement, Cohen will make an offer to buy all of the bank's stock but will have to purchase shares only if holders of at least 271,000 of the shares -- 61 percent -- agree to sell.

Neither Cohen nor major stockholders of Hemisphere Bank would discuss terms of his option to buy control of that tiny bank, which was organized as a minority bank serving the Hispanic community.

Cohen said only that he has options to buy "more than 50 percent" of Hemisphere's stock but refused to disclose the sellers or the agreed upon price.

Hemisphere has deposits of less than $11 million. Since new management took over the struggling Diplomat National bank recently and brought in several million dollars of new deposits, Hemisphere is believed to be the second-smallest bank in the District, ahead of Women's National Bank with deposits of $3.5 million.

About 28 percent of Hemisphere's stock is owned by Nestor Garcia, an Argentinian banker who reportedly has been frustrated in efforts to buy a majority interest and has discussed selling his stock. Garcia's Washington attorney and Hemisphere Chairman Leveo Sanchez could not be reached for comment yesterday; the bank's board of director's was scheduled to meet last night.

The board of D.C. National Bank also had a meeting scheduled for yesterday, but no major changes in that bank's top management are expected until after the first of the year, when Chairman S. Greenhoot Fischer retires.

Fischer announced in August he would sell his approximately 15 percent of the bank to John Safer, a prominent Virginia real estate investor. Reports to federal banking regulators show Fischer got $62 a share for his 32,000 shares, a total of $1.98 million. The stock was trading for $35 to $36 a share, roughly its book value, at that time.