The world petroleum-exporting cartel ended a three-day meeting here yesterday but failed to agree on how to combat the twin problems of inflation and the declining dollar.

For the third straight time in 18 months, the Organization of Petroleum Exporting Countries made no significant decisions. This fact alone raised the question of whether OPEC - which steered through a quadrupling of oil prices earlier in this decade - has outlived its usefulness.

This time there were no walkouts, not even angry exchanges. When it ended in the plush Intercontinental Hotel this morning, oil ministers whose views differ sharply, agreed they had had a good meeting, even a successful one.

OPEC, whose member nations control about 85 percent of the oil in international trade, is hamstrung not only by the differences among its members but by the simple fact that it operates by consensus. Nothing is done unless all 13 members agree.

As in the past, Saudi Arabia stood up to pressures from other members for a price rise. This time the pressure focused on the declining dollar, which has gone down 16 percent in value since December 1976, the last time OPEC raised prices.

As a result, there was no price increase, only appointment of a committee to study the problem. There was considerable doubt among ministers whether there would be an increase at the next meeting, scheduled to be held in Abu Dhabi next December.

"I am very satisfied," said Sheikh Ahmed Zaki Yamani, Saudi Arabia's oil minister. "The results speak for themselves."

[Unless oil prices rise early next year, there will have to be a sharp increase in the 1980s, Yamani said at a news conference after the meeting, United Press International reported.]

[Yamani said there could be an increase of one or 2 percent early next year, followed probably by two similar increases later in the year.]

For a while Sunday, it looked as though Saudi Arabia, OPEC's biggest producer, might be wavering. The sudden and unexpected arrival here of Crown Prince Fahd, the nation's strong man, was subjected to various interpretations.

But if Fahd had anything to say to Yamani, it was to hold the line. The session, which already was running overtime, was cut off when Yamani informed his fellow ministers that Saudi Arabia was sticking fast.