Defense Secretary Harold Brown yesterday gloomily portrayed a world growing ever more dangerous as old disputes "continue to fester," new ones arise and Soviet military power increases.

"Instability in the Middle East will be the rule rather than the exception for some years to come," Brown said in his assessment, given to Congress to justify $158.7 billion in new money for the Pentagon in fiscal 1981, 19.5 percent more than in fiscal 1980. Subtracting the impact of inflation, the Pentagon figures the increase in budget authority would drop to 8.3 percent.

The Soviet Union shows no sign of slowing either its nuclear or conventional weapons buildup and may even be planning to field enough forces to fight three wars at once, Brown said. By contrast, he said, the United States has "never fully acquired the agility and the mobility" to fulfill its current strategy of being able to fight one big war and one small one at the same time.

The United States could not send in enough reinforcements now to give staying power to forces deployed far away, the defense secretary said, and U.S. allies have "even been more cavalier" about such support for their own forces.

Brown became specific in his 300 page "posture statement" about Soviet threats to the sea lanes, which are the jugular veins for the West because foreign oil must move through them. He underscored to an unprecedented extent the threat from Soviet Backfire bombers armed with antiship missiles.

In other parts of his "posture statement," the defense secretary conceded that arms control efforts had stalled, and that the United States would have to keep spending more and more billions to combat a growing Soviet arsenal.

He said the Soviet arms buildup goes well beyond any reasonable need for that nation's own security, leading to speculation that the Kremlin has adopted a "three-contingency strategy" that would give it the capability of fighting as many as three wars at once.

The Backfire threat to the sea lanes has been overshadowed in past years by the U.s.-Soviet argument over whether the plane has enough range to bomb the United States and thus should be counted as a strategic weapon in arms control agreements.

The fresh attention given to the antiship version comes at a time of new administration concern over whether the United States can protect its vital supplies of oil in the Middle East and elsewhere.

"The growing Backfire threat will exceed the menace of submarines," Brown said yesterday in discussing dangers to the sea lanes.

He said that the number of Navy warships is such that the Soviets' long-range Backfire bombers armed with antiship missiles could first concentrate their fire on them, then pick off unarmed tankers and cargo ships. The Navy has 181 surface warships and 13 aircraft carriers in its active fleet.

He indicated that the United States will stick with its "barrier strategy" for combating Soviet submarines but will revamp its shipbuilding program to cope with the Backfire threat.

The barrier strategy calls for bottling up or sinking as many Soviet submarines as possible before they can get from their home ports to the open sea.

But the barrier strategy does nothing to stop the overhead threat to convoys posed by missiles fired from Backfire bombers. As a result, Brown said, the administration has decided to build more Aegis antiaircraft cruisers at the expense of lighter and cheaper FFG7 patrol frigates armed primarily to sink submarines.

But Brown warned yesterday that there is no guarantee that the Aegis cruiser would be a match for the Backfire, which could fire its weapons a long distance from the U.S. warship or tanker.

On that point, Brown said:

"We lack an adequate defense against massed bomber and missile attacks. How well we can now counter the threat with land-based and carrier-based aircraft and Aegis-equipped ships remains to be seen."

The Pentagon estimates that the Soviets already have built about 100 Backfire swing-wing bombers, half of them assigned to patrolling the oceans and the other half assigned to land warfare. The Soviets are building 30 new Backfires a year, half of them assigned to the antiship role.

In discussing other Soviet weapons, Brown confirmed that the Soviets have built a titanium alloy submarine that "can probably operate at greater depths than other" attack subs and "has demontrated high sustained speed." Brown said the sub recently was deployed out of its home are for the first time. The Washington Post, in reporting on the submarine's existence May 18, said that the sub had been clocked at 40 knots, faster than any U.S. submarine.

The defense secretary discounted reports by retired Air Force Maj. Gen. George Keegan and others that the Soviets were on the verge of developing, as a weapon, a beam of charged particles that could knock down incoming missiles.

Besides crediting the Soviets with a continued buildup of their nuclear and conventional forces, Brown said that these forces are changing in character to give the Kremlin a longer reach around the world.

Brown said the United States "can only speculate as to whether the Soviets have actually adopted a three-contingency strategy just a decade after we abandoned it."

The reference was to the 2 1/2-war strategy embraced by President Kennedy in the 1960s, when Brown served as Pentagon research director and then secretary of the Air Force.