France and Japan, lending discreet support to the United States, have quietly pledged not to buy any excess Iranian oil freed by the U.S. decision to boycott Iran's petroleum exports, authoritative diplomatic sources said here today.

West Germany had made the same pledge publicly. The three countries and the United States jointly account for about three quarters of the non-Communist world's total demand for imported oil.

The moves by the three allied countries suggest that Iran may have difficulties in cashing in on its extra suplies as heavily as it had hoped. Iranian officials had said that excess oil would be sold for $45 per barrell, or more than twice the official OPEC price, on the spot markets.

The show of solidarity with the American action against Iranian oil could serve the double purpose of putting economic pressure on Iran over the U.S. hostages in Tehran and of finally reducing, at least for the moment, spot oil prices.

julius Katz, assistant U.S. secretary of state for economic affairs, said that the actions by the three allied countries were spontaneous and had not been requested by Washington.

The United States and France had been in the forefront of countries trying for some time to break the pattern of rising prices for spot market oil -- oil that is not tied up in long term contracts at set prices.

Iran has been the leading force in driving prices up. It was known to be demanding that companies with contracts for Iranian petroleum at set prices nevertheless pay the higher prevailing free market rates for a portion of their Iranian purchases or face the prospect of being cut off altogether.

The moves of France, Japan and West Germany also tend to counter the impression that Iran was succeeding in driving a wedge between the United States on one side and Western Europe and Japan on the other.

The impression had been fed by the unwillingness of some countries especially France, West Germany and Italy -- to help enforce the U.S. freeze on Iranian dollar deposits both in the United States and abroad.

Iran today tried to lure other Western allies to break ranks with the Americans when Acting Foreign Minister Abol Hassan Bani-Sadr, who is also in charge of economic affairs in the Tehran Revolutionary Council, announced that payments for Iranian oil could be made in currencies other than the U.S. dollar.

The Western solidarity over oil could also have a psychological effect on the participants at the next OPEC oil-price setting conference in Caracas, Venezuela, in December.

In another attempt to impress OPEC, the United States pressed today for the industrial West to set its energy house in order before the OPEC meetings.

The U.S. delegation to a meeting here of the governing board of the International Energy Agency, grouping the 20 leading non-Communist oil consumers except for France, urged that the organization hold a ministerial-level meeting in December in advance of OPEC.

The purpose would be to announce a series of energy-saving measures. "The objective," said board chairman Neils Ersboell of Denmark, "is to reach agreement on a much tighter grip on our own oil demands so that we can answer OPEC demands" that the West can save more oil.

The initial indications were that the other countries would accept the American proposal for an early meeting designed to counteract OPEC pretexts for a stiff new price rise.

One reason American leadership is now apparently more welcome than it has been is that the other countries have finally started to accept the seriousness of the United State's own energy-saving efforts. For a long time, the Europeans and Japanese were pointing their fingers at the Americans over congressional failure to heed the Carter administration's exhortations to pass energy measures.

Katz noted that U.S. gasoline, consumption, for example, is 7 to 8 percent less than it was last year.

The inclination of even the oil world's moderates has clearly been toward higher prices. An important European business figure just back from a tour of the Persian Gulf said that one emir that had told him there had been a meeting recently at which the Saudi Arabians agreed with the smaller countries that there should be a sizeable price increase. The Saudis have been the main moderating force on OPEC prices.