An Arab and a Kansas bank were surprise bidders at the Treasury's third gold auction of the year yesterday. The average price per ounce was $185.16.

Previous auctions have been dominated by German and Swiss banks, making yesterday's successful bids by the Bank of Oman. Ltd. of the United Arab Emirates and the Exchange National Bank and Trust Co. of Atchison, Kans. a novelty. In fact, Treasury officials said, this marks the first time an Arab bank has ever bid directly for U.S. gold. Normally a European bank acts as an agent.

The Bank of Oman, located in the sheikdom of Abu Dhabi, is a private stock corporation. It has 23 branches in the Middle East and London. The bank, which has participated frequently in International Monetary Fund auctions, yesterday bought 100,000 ounces of U.S. gold, worth $18.5 million. The Kansas bank purchased 1,200 ounces at $187 an ounce, while Adachi Gold Co., Ltd. of New York was the high bidder, paying $189 an ounce for 4,000 ounces.

While official results were not available yesterday, other apparently successful bidders were Johnson Matthey Bankers Lts., of London, official smelters for the crown, who bought 100,000 ounces; the Dresdner Bank of West Germany, which bought 96,000 ounces; Credit Suisse, Bank Leu of Zurich, Deutsche Bank, and Sharps Pixley, metals traders.

There were 29 bidders at yesterday's sale making bids to purchase 1.3 million ounces of gold, although only 300,000 ounces were offered. Gross proceeds from the sale were $55.5 million, of which 12.7 million will be used to pay off gold certificiates held by Federal Reserve banks. The next auction is set for Aug. 15.