The Reagan administration has signaled it will scrap President Carter's biggest NATO initiative: getting alliance members to commit themselves to increase defense spending by at least 3 percent a year after allowing for inflation.

In an apparent policy shift expected to delight NATO partners, particularly West Germany and Britain who contend an inflexible 3 percent increase is too demanding, Defense Secretary-designate Caspar W. Weinberger said he does not believe in holding allies to such a rigid standard.

While stressing NATO and other friendly countries must join the United States in doing more to shore up their defenses and keep world sealanes open, Weinberger kicked President Carter's 3 percent standard toward the ash can with these words at his Senate confirmation hearing on Tuesday:

"I don't know that it is particularly useful to demand or talk about specified or fixed percentages" of increases in friendly countries' defense budgets. Instead, he indicated the Reagan administration will settle for a generalized commitment.

"It is in everyone's interest in NATO and Japan and elsewhere to join in this effort" by the United States to right the adverse military balance between the West and the Soviet bloc, Weinberger said.

"It's a mutual enterprise in which everybody has exactly the same interest," he added. "I don't think it's an effective measure simply to look at how much or what percentage. I would like to see the end result. I don't believe in these fixed percentages. I think sometimes they may be too low and sometimes they may be high enough to involve some waste. What I would like to look at is the end product; are we getting the strength we need? There's no doubt that requires increased percentages, but by the same token I think we will get a far more effective result if we look at what we are getting for it."

President Carter and Pentagon leaders hailed the commitment of NATO members at the summit meeting in Washington in May 1968 to increase their defense spending by 3 percent a year as a major breakthrough. Administration officials had described the commitment as the big achievement in NATO of the Carter years.

However, economic pressures on West Germany, Britain, Belgium, Denmark and The Netherlands have impelled those countries to back away from the 1968 pledge, much to the distress of the Carter administration. Weinberger's testimony indicates the new administration is resigned to fluctuating defense budgets abroad.