President Carter called an unusual, three-hour meeting yesterday afternoon to try to thresh out some of the differences between the Departments of Labor and Health, Education and Welfare over competing welfare reform proposals.

The need to simplify existing welfare programs and make them fairer and more logical was a major Carter campaign issue. On Monday he is sheduled to outline at least the broad, overall direction his reform proposals will take.

But what he will say has not yet been written. Speechwriter James Fallows was one of 21 persons, including Mrs. Carter, Labor Secretary Ray Marshall and HEW Secretary Joseph A. Califano, Jr., who sat in on yesterday's meeting.

Sources close to the meeting said the biggest single unresolved issue centers around how much emphasis the government should place on guaranteeing jobs for household heads who are able to work.

Earlier this month the administration was considering five major reform options, including a HEW proposal emphasizing cash grants to poor families in a manner similar to the family assistance plan advanced by the Nixon administration several years ago, and a Labor Department proposal emphasized guaranteed jobs, particularly public service jobs.

In an almost endless series of meetings since then, sources said, one option calling for expanding and simplifying existing programs has been dropped, and Labor and HEW have managed to resolve some of their differences.

Labor Department sources said Marshall wants to take 725,000 public service jobs the administration hopes to create by the end of the fiscal 1977 as part of the economic stimulus package and re-target them to those on welfare.

Under existing plans, 6000,000 of those jobs are intended for people on welfare or unemployment areas to stimulate local economics.

Marshall believes that would take care of most of the million-plus welfare recipients who could reasonably be expected to work, the sources said.

Other sources indicated that Labor and HEW have anywhere from two more weeks to another month to see how many of their remaining differences they can reconcile. Both their proposals now call for a mixture of jobs and cash assistance, but Labor tends to emphasize the jobs, while HEW tends to emphasize the cash assistance.

One source said Mrs. Carter did not say anything while she was in the meeting, but gave the impression, she was 'soaking up all of this,' perhaps intending to talk to her husband about it later.

The White House acknowledged yesterday that at least 73 transition aides to the President-elect received $200 to $300 cash advances in November and December for more than 600 trips to Annapolis which were never taken.

Deputy press secretary Rex Granum said the advances were a cover for personal loans to the staffers, granted because their paychecks were delayed 'for six weeks or more,' and that all of the money has been returned.

The destination Annapolis was a code to show that the money was really a loan, the White House said.

In a Friday interview with the board of directors of the Radio Television News Directors Association, Carter said the United States needs a nationwide water conservation program, and that 'will be, maybe, the next thing after energy.'

His proposal to put an additional tax on crude oil, part of his national energy plan, will add about 7 cents per gallon to the price of gasoline, he disclosed.

And he said his energy advisers tried to set targets for the diminished use of gasoline so that 'there would be at least a 50 per cent chance' that his proposed gasoline tax, triggered by exceeding those consumption targets would not have to be imposed.

Carter told the news directors he has not 'gone yet to meet personally with oil (company) executives to ask them if they would espouse the program in its entirely as a patriotic gesture. I think it would be very good for them and the country if they would.'

He also said he still gets 65,000 to 85,000 letters a week, and his 9-years-old daughter, Amy, gets 2,000 letters a week.