The military services have been told that it will take a $27 billion cut in spending authority, about $5 billion more than previously reported, to achieve the savings President Reagan has ordered over the next three years, Pentagon sources said yesterday.

Under the new marching orders the Air Force would have to absorb most of the cut, although Reagan's long-awaited and still-unmade decisions on building a new bomber and deploying an MX missile could bring back some of this money.

Deputy Defense Secretary Frank C. Carlucci, in a secret memo giving the services the bad news about the cuts, also reflected the Reagan administration's sense of urgency about being prepared to fight to protect oil sources in the Persian Gulf. He forbade economies in accounts related to the rapid-deployment forces that would be sent there.

"All services should continue with the major increase in Southwest Asia and Rapid Deployment Force capabilities initiated in the fiscal 1981 and 1982 budgets as amended by the Reagan administration," Carlucci ordered.

The Navy must go ahead with preparing a hospital ship for Persian Gulf casualties, Carlucci said, and the Army and Air Force, while economizing elsewhere, must "increase their Ras Banas construction efforts."

Ras Banas is a finger of Egyptian land on the Red Sea pointing toward Saudi Arabia. The Reagan administration, like the Carter administration before it, sees Ras Banas as a key staging area for any American rapid deployment force sent to make a stand in Southwest Asia.

Millions of dollars have been earmarked to improve the austere Egyptian airfield at Ras Banas so long- range B52 bombers, aerial refueling planes and cargo aircraft can wing in and out of there.

Carlucci, in another part of his memo, warned the services against cutting out money earmarked to warehouse equipment in West Germany to support reinforcing divisions that would be sent to Europe in a crisis.

"To keep pressure on our NATO allies to increase their defense efforts, it is important that we continue to plan to meet our commitment to fielding a 10-division U. S. force in the central region promptly" by putting the equipment for the extra five divisions in Germany ahead of time, Carlucci said.

To make peacetime life better for American service families in Germany, Carlucci continued, plans to improve their housing should not become a casualty of the new budget cuts, either.

He also warned the services against gutting the unglamorous accounts that finance repairs of tanks, ships and aircraft already on hand. But, in a decision congressional committees are likely to contest, plans to stockpile enough ammunition and spare parts to keep soldiers fighting for 60 days without resupply from distant depots can be stretched out under the Carlucci fiscal guidance.

The Pentagon on its own, Carlucci told the services, has already decided to stretch out additional funding to gear up American industry for wartime production. This fund will go from the planned $463 million for the next five years down to $100 million.

The $27 billion cut over the next three fiscal years would not be a cut at all, but a reduction in the large increases Reagan had proposed earlier for the Pentagon. Its budget would still go up. The $27 billion refers to spending authority, the right to obligate funds.

That large a cut in authority would be required to produce the $13 billion cut in spending over the next three years, Reagan's budgetary objective. Pentagon officials previously had estimated that a $22 billion cut in authority would do the job.

Here is how the services' budgets would be cut under the Pentagon's new plan to cut military spending by $13 billion over the three-year period fiscal 1982 through 1984, expressed in total obligational authority:

Army: Fiscal 1982, from $52.6 billion to $50.7 billion; fiscal 1983, from $58.8 billion to $56.2 billion; fiscal 1984, from $65.8 billion to $63.9 billion. This is a three-year cut of $6.4 billion.

Navy: Fiscal 1982, from $70.8 billion to $68.3 billion; fiscal 1983, from $79.8 billion to $77 billion; fiscal 1984, from $89.8 billion to $87.8 billion. This is a three-year cut of $7.3 billion.

Air Force: Fiscal 1982, from $67 billion to $64.7 billion; fiscal 1983, from $75.8 billion to $70.1 billion; fiscal 1984, from $85.3 billion to $79.6 billion. This is a three-year cut of $13.7 billion.

All told, total obligational authority would be reduced for fiscal 1982, 1983 and 1984 by a total of $27.4 billion, with the Air Force absorbing half of it. The White House said on Saturday that it wanted military spending reduced by $2 billion in fiscal 1982, $5 billion in fiscal 1983 and $6 billion in fiscal 1984.