Treasury Secretary James A. Baker III has managed, in his first three weeks in office, to be nice to the Federal Reserve Board, hold plenty of talks with business groups upset about possible changes in the tax code, testify before Congress as often as three times in one week and attempt some reorganization of the Treasury Department staff.

Part of the realignment will involve the position now held by Beryl Sprinkel, the undersecretary for monetary affairs who has been nominated by the president as chairman of the Council of Economic Advisers.

It may be two or three weeks before Sprinkel is confirmed by the Senate for his new job.

Baker is working on a reorganization of Sprinkel's old job, which included many international assignments.

It is likely that the undersecretary position will be retained but the responsibilities will be different. Treasury insiders say that Deputy Secretary Richard G. Darman and David Mulford, assistant secretary for international affairs, both want to be in charge of the Treasury's international duties.

The official word is that the naming of the new undersecretary will await the resolution of the organizational problem, and it is as yet unclear with whom the international responsibilities will lie. THE HEARING-ROOM REPORT . . .

Baker has kept his cool so far when testifying before Congress on issues such as tax simplification and the Federal Reserve Board's monetary policy.

He has repeatedly refused to divulge what options in the Treasury tax proposal he is considering eliminating or modifying, although he is known to be sympathetic to business concerns.

Known as a masterful negotiator while chief of staff at the White House, Baker told Congress that the tax simplification plan was a massive undertaking and "we're not going to get it, quite frankly, without bipartisan support."

Baker, while testifying on Capitol Hill last week, also faced a barrage of questions about the administration's proposed budget. At one point he was asked why the administration was insisting on a 5 percent across-the-board pay cut for federal workers, including the U.S. Secret Service.

Baker said he did not consider the Secret Service overpaid, but added, "That's not the same as saying I don't think that there are people in government who are overpaid . . . . We do think there are some in the federal government who are overpaid; there are some who work harder than others just as there are people in private industry who are overpaid."

Under sometimes heated questioning by Rep. Jack Kemp (R-N.Y.), a proponent of more restrictions on the Federal Reserve, Baker repeatedly said that the Fed's monetary policies were adequate.

He would criticize the Fed publicly only "if we can't get together privately," Baker said. But he said the criticism ought to be "a two-way street. If they're going to criticize us on our fiscal policy we should be able to criticize them" on their monetary policy. WAITING FOR APPROVAL . . .

Margaret Tutwiler, formerly an assistant to Baker at the White House, is still waiting to hear when she will be confirmed in her new post as assistant secretary for public affairs and public liaison. She has brought with her from the White House Caron Jackson, Julie Brink and Karen Groomes, whose official duties have not been decided, a Treasury spokesman said.