Delta Airlines yesterday placed the largest single aircraft order in commercial aviation history when it purchased 60 new 757 jets from Boeing Co. cfor $3 billion.

With that one stroke, Delta more than doubled the number of firm orders Boeing has for the new 757 and at the same time deprived McDonnell Douglas of the kind of order it would need to launch a similar, competitive airplane it calls the DXCC. Douglas officials thought the DCXX was still in the running with Delta as recently as a week ago, according to industry sources.

The 757 is one of the two new fuel-efficient jets Boeing is marketing and the one that was doing least well in advance sales. It will be a narrow-body, twin-engine craft with about 180 seats and a range of slightly more than 2,000 miles. Boeing regards it as a successor to the immensely popular 727 and so does Delta.

"The 757 fits well into our short- to medium-haul markets," said David C. Garrett Jr., Delta's president and chief executive officer. More than 70 percent of Delta's flights are 60 miles or less, he said.

The 60 jets will be delivered over a six-year period starting in 1984. Garrett said that by the time the first plane is delivered, even newer, more fuel-efficent jet engines will be available for the 757 than the ones on the first model, which is scheduled for delivery in January 1983.

In the next month, Delta will choose an engine for its 757s from models offered by General Electric Co., United Technologies Corp.'s Pratt & Whitney Aircraft Groups and Rolls Royce. Garrett said that Delta expects the 757 with the new engines to be 30 percent more fuel efficient than its present fleet.

Garrett said Delta has not decided whether to order the 757 with a two-man or three-man cockpit and that he did not have to make that decision until 1981. oThe Air Line Pilots Association has been conducting a vigorous campaign against two-man cockpits, charging they are not safe as three-man cockpits. Statistics comparing the accident rates of planes with two-or three-man crews indicate there is nothing to choose exept whether to pay the extra salary.

Delta is one of the few airlines that has continued to prosper despite the recession. It recenty reported third-quarter earnings of $26.3 million compared with $12.6 million a year earlier. Garrett said Delta expects to finance between 80 percent and 85 percent of the purchase price of the 757s from internally generated cash and will need to borrow only between 15 percent and 20 percent. He said he sees no significant change in Delta's debt-to-equity ratio over the period of the purchase.

Garrett said that Delta had considered a number of airplanes, but that the 757 "comes out head and shoulders ahead of anything that's available right now."