President Carter's plan to increase the nation's military budget by 3 per cent next year is now regarded as too big a jump by his budget advisers.

They are fighting to hold the Pentagon's fiscal 1979 budget down to $125 billion rather than the $130 billion projected by Defense Secretary Harold Brown.

The President, in a policy directive not made public but described by White House officials, recently called for a real annual increase of 3 per cent a year in future Pentagon budgets.

The President's Office of Management and Budget figures that would be too big an increase for the total budget. They are recommending instead that the 3 per cent hike apply only to programs related to NATO, not to the entire defense budget.

If only NATO-related programs were increased by 3 per cent, OMB analysts are arguing, the total Pentagon budget would rise, after allowing for inflation, only about 1 per cent between fiscal years 1978 and 1979.

Congress appropriated $116.6 billion for the Pentagon for fiscal 1978, the budget year which began Oct. 1. Allowing for inflation, Pentagon officials figure a real increase of 3 per cent would push the fiscal 1979 budget up to around $130 billion.

Carter wrote the Democratic Platform Committee on June 10, 1976, that "Without endangering the defense of our nation or our commitments to our allies, we can reduce present defense expenditures by about $5 to $7 billion annually."

Defense Secretary Brown has been contending that Carter's pledge has been fulfilled because the Ford defense budget for fiscal 1978 would have totaled $123 billion, compared with the Carter budget of $166.6 billion.

Looking ahead to the fiscal 1979 defense budget now in preparation, Pentagon officials are predicting it will end up "at least" $5 billion to $7 billion under what Ford had predicted for that year (Ford had predicted $133.1 billion in budget authority for fiscal 1979. The Pentagon, apparently allowing for inflation, puts that Ford projection at $136.4 billion).

Brown, according to his aides, still has a thick stack of budget analyses to go through before arriving at a final recommendation to Carter on how high next year's Pentagon budget should be.

Brown, administration officials said, is not battling with OMB officials directly over whether the 3 per cent increase should be general or limited to NATO. That decision, sources said, will be left up to Carter.

Sen. William Proxmire (D-Wis.) said Wednesday that a Pentagon budget of over $130 billion for Fiscal 1979 would break the President's pledge to reduce defense spending by $5 billion to $7 billion annually.

Congress made half of the total cut in fiscal 1978 that enable Carter to fulfill his campaign pledge for that year, Proxmire said.

The senator added that before the United tates adds substantialy to its current annual contribution of $19.6 billion to NATO, "we should insist that the Europeans match U.S. contributions."

At present, Proxmire continued, the U.S. contribution to NATO works out to $523 per person in this country, compared with $263 per person in Germany and $201 per person in Britain.