Despite strong complaints by President Carter, the government of Prime Minister Menachem Begin is reportedly preparing three new Israeli settlements in the Arab heartland of the occupied West Bank of Jordan.

The Israeli newspaper Maariv said yesterday that the three new sites, all near Nablus, are now military out-posts occupied only by soldiers who are clearing the ground. But upon completion, the paper said, the sites, with the approval of the Israeli government, are to be turned over to civilian settlers.

Coming almost on the eve of Egyptian President Anwar Sadat's trip to the United States and only a day after President Carter publicly reminded Israel of its commitment to forego new settlements, the report is almost certain to cause diplomatic complications and strains between Washington and Jerusalem.

Asked about the report, a spokesman in Begin's office said that development of the sites had been approved earlier by the Cabinet.

Development of these three sites, Mcariv said, completes the first stage of the settlement plan of the Begin government, which includes a total of 16 settlements in the West Bank highlands and 23 in the Jordan River Valley.

Carter, in his press conference Monday, repeated the U.S. contention that establishment of civilian settlements in the occupied territories is illegal under the Geneva Convention and is an obstacle to peace in the Middle East.

In a message to Begin last week and in his press conference, Carter said Begin and Foreign Minister Moshe Dayan had given him a "commitment" that "no new settlements would be authorized by the government" and that any increase in the number of Israeli settlers in the occupied territories would be through "expansion of existing settlements as much as possible within the aegis of the military."

Israel has interpreted the Geneva Convention as prohibiting civilian settlements only if they involve the displacement of the local population - something it says is not happening.

Carter's message to Begin was prompted by reports that a civilian settlement was being established at Shiloh. This site, 10 miles south of Nablus, the largest city in the West Bank, is where, according to biblical tradition, the ark of the convenant was kept before King David moved it to Jerusalem.

Carter said at his press conference that he had received assurances that the Israeli government had authorized only an archeological project at Shiloh, and not a settlement.

The two dozen people camping at Shiloh, however, tell visitors that they are there to stay. So far the government has made little effort to remove them.

The three new sites are also in the vicinity of Nablus, 30 miles north of Jerusalem. They are Tabuah, six miles south of Nablus; Silt a Dahar, to the northeast and Heres to the west.

According to Maariv, Heres is to be settled by the Gush Emunim movement, a strongly nationalist Jewish group dedicated to the eventual annexation of the West Bank The paper said it was not clear who will settle the other two sites.