In an abrupt turnabout, Interior Secretary James Watt yesterday submitted three plans to the Commission of Fine Arts for placement of an American flag and heroic statue near the Vietnam Veterans Memorial. He requested that the commission "not overlook the feelings of the Vietnam veterans in consideration of esthetic and architectural concerns."

At the same time, Watt sent letters to Sens. Charles McC. Mathias Jr. (R-Md.) and John Warner (R-Va.)--both deeply involved in all phases of shepherding the controversial memorial to completion--thanking them "for creating the political consensus that will allow us to move to the final phase."

On Friday Watt had said he didn't plan to make the submission for the Feb. 8 commission meeting and that it would take more than a year to develop a political consensus on where to place the flag and statue. According to an Interior Department spokesman, Mathias and Warner talked with Watt and "reached a consensus."

Rep. Morris K. Udall (D-Ariz.) also advised Watt to move ahead quickly rather than wait, the spokesman said.

"We'll see what happens . . . We are a review body and we react to design submissions," said J. Carter Brown, chairman of the Commission of Fine Arts. "We'll approach it with an open mind and see if any of the three seems satisfactory."

The commission, a presidentially appointed review body, must approve any plan for the flag and statue under the legislation that authorized construction of the Vietnam memorial on the Mall. Approval of these additions and their placement is also needed from Watt and the National Capital Planning Commission.

Warner said in a statement yesterday he was "extremely pleased" at Watt's action, which was "suggested by Sen. Mathias and myself . . . The proper forums to determine the positioning of these two important additions to the memorial are the fine arts commission and the National Capital Planning Commission rather than by Congress."

The V-shaped, black granite memorial, dedicated in a ceremony attended by 150,000 Vietnam veterans last November, was opposed by many veterans as unheroic. These opponents were able to reach an agreement with the Vietnam Veterans Memorial Fund, which originated and built the memorial, to have the flag and statue added.

Washington sculptor Frederick Hart designed a sculpture of three walking combat troopers, heavily armed and with somewhat surprised expressions on their faces. Everybody agreed on it, but then a battle developed over where the flag and statue would be placed.

Those who disliked the memorial to begin with demanded the flag and statue be placed prominently. Such a plan was submitted to the fine arts commission last October, which turned it down on esthetic grounds and suggested they be placed in an "entryway" plaza some distance from the memorial in the direction of the nearby Lincoln Memorial.

The plan for prominent placement, the entryway plan and one other were sent to the commission yesterday.

"Any of the three plans is fine with us . . . just so that we can get the job done," said Jan C. Scruggs, head of the memorial fund.

But there may be a fight at the Feb. 8 meeting. "I'm afraid Watt's action just gives the commission the opportunity to thwart the will of Vietnam veterans. They commission members operate in contempt of Vietnam veterans," said Milton R. Copulos, a veteran who opposed the memorial design and played a key role in obtaining the additions.

Copulos said that at the October fine arts commission meeting "it was only a very small segment of the arts community that objected" to the prominent placement of the flag and statue.

In his letter to the commission yesterday, Watt said he still favored the plan that would give prominent placement to the flag and statue, as he had in October.

"We feel that this option . . . fully meets the commitment to reach a compromise on this issue," Watt wrote. "We believe that this option truly honors all those who served their nation in the Vietnam war."