The Reagan administration is "seriously concerned" about a third test flight last week of a Soviet ICBM that may violate the SALT II treaty, according to a top-ranking government official.

The May 4 test came as American officials were trying through diplomatic channels to determine from the Soviets whether a violation had occurred during two earlier tests of the small, solid-fueled missile.

"Nobody has any conclusive information" on the latest shot, one government official said yesterday. He added that there were "anomalies" in the data collected that might indicate either that the Soviets had tried to hide electronic information about the test or that U.S. monitoring systems had not functioned properly.

The test, like the earlier ones, was held at night "so that we could not see the mobile launcher," according to one source familiar with intelligence data.

In Geneva, American representatives at the Standing Consultative Commission, which monitors adherence to U.S.-Soviet arms treaties, have raised questions about Soviet missile testing, according to Washington sources.

And just days before the May 4 test, the United States took an unusual step by putting a series of questions about the two earlier tests to Soviet Ambassador Anatoliy F. Dobrynin. No response has been received, according to government sources.

Answers received in Geneva and from the questions given Dobrynin will be included in an interagency verification committee study ordered by President Reagan and directed by national security adviser William P. Clark. That study is to determine whether SALT II has been violated, and if so, what the U.S. response should be.

Last week's test, according to one conservative source on Capitol Hill, was Moscow's "contemptuous response" to the administration's questions. Conservatives have urged the administration to go public with information about alleged Soviet SALT II violations before new strategic arms negotiations in Geneva.

Under SALT II, both the United States and the Soviet Union agreed to limit themselves to one new ICBM. Although the treaty has not been ratified, both sides have said they will respect its provisions.

Last October, the Soviets tested a large, solid-fueled ICBM which, Dobrynin later said, was the one permitted under SALT II. Then on Feb. 8, the Soviets test fired the smaller ICBM and later told the United States it was a modification of an older missile, the SS13. The smaller missile was tested again in March and failed.

Some U.S. analysts said the second missile exceeded the modification limits set by SALT II. Reagan administration officials called the data "somewhat ambiguous" and said they would await another test to see if they could prove a violation.