Senior U.S. and allied diplomats met for the third time in a little more than a week yesterday and reported no movement in their effort to hammer out an agreement on East-West trade that could replace the Reagan administration's Soviet gas pipeline sanctions.

The unusual sequence of high-level negotiations is now believed to have narrowed differences over an eventual accord. U.S. and European diplomats did not rule out the possibility that an agreement acceptable to all the governments involved had been reached, but said they would be surprised if further meetings were not necessary. No such meetings have been scheduled.

The sanctions issue has become one of the most troubling for the Atlantic alliance in years and there were signs of growing irritation yesterday among European diplomats.

"We don't know what the U.S. administration needs. We do know what we can do, but there isn't much room to limit or change that consensus," one senior European diplomat said before yesterday's meeting. "We are not waiting in frenzied expectation."

A senior U.S. official said the negotiating process seems to have brought home to the Europeans that "the president is serious" about his sanctions policy.

"I think something will happen, but the president doesn't have to be in a hurry," this official said.

The comments appeared to reflect the tenseness of the negotiations that have been carried out under the direction of Undersecretary of State Lawrence S. Eagleburger, negotiations that have been tightly held by the State Department, sparking something of a political guerrilla war by some elements within the administration who advocate a hard-line position on trade with the Soviets.

Eagleburger is pictured by diplomatic sources as having pressed continually for more changes and refinements in the positions put forward by European, Canadian and Japanese diplomats, without indicating what might prove acceptable to the administration.

"There is a piece of paper that has been through several stages," one European diplomat said. "It deals with general principles and guidelines rather than legally binding limits. A step three millimeters to the left or the right and another company is in trouble or trade is restricted. What we don't want is a trade war. We want ground rules."

The negotiators appear to be trying to develop carefully defined instructions for a broader study of major issues in East-West trade that eventually could produce a general policy to be adopted by NATO members and Japan.

Along with the outlines of the study, the diplomats are believed to be discussing a moratorium on Soviet energy purchases and the sale of high-technology items to Moscow.

The combination, it is hoped, might prove sufficient to allow the administration to end or modify its sanctions policy, which cuts off companies that ship U.S.-made or -licensed products for the pipeline from any other U.S. gas- or oil-related goods. Companies in Italy, France, Britain and West Germany have been placed under the sanctions; several of them are believed to be feeling a financial bite.

Italian Prime Minister Giovanni Spadolini is to arrive in Washington tonight and is to meet with Reagan on Wednesday. This would provide the next forum for discussion of the sanctions issue.

The Italians are known to be particularly upset at the impounding in New York of a number of key parts for an Algerian gas pipeline.

"You tell us to develop alternatives for Soviet gas. For us the alternative is Algeria, and then you impound the equipment needed for Algerian gas," Italian Ambassador Rinaldo Petrignani told reporters last week in anticipation of the Reagan-Spadolini talks.

The Reagan sanctions cover oil and gas equipment for any source, not just the Soviet pipeline.