The last remaining obstacle to Senate confirmation of Richard C. Holbrooke as U.S. ambassador to the United Nations fell yesterday as Sen. Charles E. Grassley (R-Iowa) dropped his "hold" on Holbrooke's nomination and switched it to three other ambassadorial appointments.

With other senators quietly abandoning efforts to hold Holbrooke hostage for unrelated concessions by the administration, Grassley's action appeared to clear the way for Holbrooke's long-stalled confirmation before Congress leaves on a month-long recess at the end of this week. A Senate vote is scheduled for this morning.

But it spelled trouble for three of President Clinton's other ambassadorial choices: A. Peter Burleigh for the Philippines, Carl Spielvogel for Slovakia and J. Richard Fredericks for Switzerland. Burleigh, a 32-year Foreign Service veteran, has been filling in at the United Nations pending Holbrooke's confirmation.

In a speech to the Senate, Grassley indicated he had no more objection to these three than he did to Holbrooke but rather was using them as hostages to force the State Department to protect whistle-blowers and reach an accommodation with Linda Shenwick, an employee who complained to Congress about U.N. waste and mismanagement.

Since putting the "hold" on Holbrooke in late June, Grassley said, he had made "some progress" in terms of "heightened awareness and assurances of a fair process" from the State Department in the Shenwick case. But he said he wanted to "increase my leverage" by stalling the three other ambassadorial nominations.

"My interest in this matter is simple," he said. "Congress cannot function . . . if government employees cannot communicate with Congress about wrongdoing."

Senators can signal their intention to delay a bill or nomination by imposing a "hold." Senators often use the practice to win concessions on other matters.