Lawyers for both sides in a Justice Department suit alleging biased hiring and promotion policies by Fairfax County completed their final arguments yesterday, winding up a three-day trial in federal court in Alexandria.

U.S. District Court Judge Albert V. Bryan Jr. took the case under advisement, and is expected to issue a ruling in the next few weeks.

The suit, filed by Justice last December, charges that the populours, affluent county practices widspread sex and racial discrimination, an allegation Fairfax officials deny.

Such discrimination "is simply not the way of doing business in Fairfax County," assistant county attorney Jack Gould argued yesterday as the trial drew to a close.

Gould said that the county "has tried to do a good job" in hiring minorities.The government, Gould said, "doesn't take into consideration" that Fairfax implemented its own affirmative action program on Jan. 1, 1978, which increased the number of mimority applicants. With that increase, "the numbers (of applicants) that come to you that are less than fully qualified increases. That's just a fact of life," said Gould.

Nearly a dozen current and former Fairfax emploues told the court on Monday that the county discriminated against blacks and women in its hiring and promotion policies.

Countering their testimony, Gould argued yesterday that "ladies who complain of lack of upward mobility does not amount to sex discrimination in the county."

But Gould also said that efforts to hire and promote blacks and women before 1978 were less "vigorous" than at the present time and that the county's personnel record keeping was "spotty."

Justice Department attorney James Angus, in his closing statement, said the government was "very skeptical" at first about tackling the case, which involved nearly 50 witnesses and cartons of evidence.

Angus criticized Fairfax officials for "failure to provide the United States access to sources of information." Gould denied that charge, saying the county cooperated fully with the government.

In February, U.S. District Court Judge Oren R. Lewis, presiding over preliminary hearings in the case, biocked the government from cutting off about half a million dollars in federal funds to the county and criticized the Justice Department for not having enough facts to support its case.

The suit asks for a general injunction against Fairfax to stop the alleged discrimination against blacks and women. It also asks that the county provide "numerical relief" by increasing the percentage of blacks and women in both traditional and nontraditional job categories.

Angus pointed out that in 1978, the Fairfax police department hired 60 people-among them only four blacks and one woman.