Government prosecutors told a federal appeals court yesterday that prison guards took reasonable security precautions "in an extraordinary case" last July when they seized a three-page document from John W. Hinckley Jr., who is accused of the attempted assassination of President Reagan.

A lower court judge ruled last November that the document, which is under court seal, was taken illegally from Hinckley in violation of his constitutional right to privacy and could not be used as evidence against him.

Hinckley's trial, which was scheduled to begin in January, has been delayed indefinitely while prosecution and defense lawyers ask the courts to resolve questions about the seized document and other evidence to be used in the case. Those questions could go to the U.S. Supreme Court before the trial begins.

Assistant U.S. Attorney Michael W. Farrell argued yesterday in the appeals court that prison guards, concerned about Hinckley's prior suicide attempt and alerted by words they saw on the confiscated papers, were justified in their decision to take the document during a routine inspection of Hinckley's prison cell at the federal correctional institution at Butner, N.C., last summer. Hinckley currently is confined at the army stockade at Fort Meade, Md.

The government has said in court papers that words on the document suggested evidence "of a possible threat to the president and other serious crimes . . ."

Prosecutors want the appeals court to overturn the lower court decision and allow the document to be used at Hinckley's trial to help counter Hinckley's defense that he was insane last March 30 when he shot Reagan and three others outside the Washington Hilton Hotel.

Farrell also urged the appeals court judges to reverse Judge Barrington D. Parker's additional ruling that FBI agents violated Hinckley's constitutional rights when they questioned him without a lawyer present several hours after his arrest. Parker blocked the government from using those statements at Hinckley's trial and prohibited any trial testimony from federal agents about the questioning session.

The government has contended that most of the questioning of Hinckley concerned basic, non-incriminating, biographical information. However, under questioning by appeals court Judge Patricia M. Wald yesterday, prosecutor Farrell acknowledged that agents also asked Hinckley what he had been doing during the year prior to the shooting.

In addition to Wald, Chief Judge Spottswood W. Robinson III and Judge J. Skelly Wright heard the arguments in the appeals court yesterday.

Defense lawyer Gregory Craig told the appeals court that the Butner prison guards were looking for contraband when they searched through Hinckley's private papers and only later tried to justify the seizure of the documents with a claim of concern about another suicide attempt. The defense, arguing there were no formal guidelines at the prison for examination of inmates' writings, contends guards should not be permitted to make independent decisons about when documents should be read.

Robinson said at the close of oral arguments yesterday that the judges would issue their decision in the case "as expeditiously as possible."