Three low-income Arlington women sued the Department of Agriculture and Virginia welfare officials yesterday, trying to force the federal agency to change a food stamp program regulation which they claim is "unfair", bad policy, discriminatory and illegal."

Legal Services of Northern Virginia filed the suit in federal District Court in Alexandria on behalf of the women and almost 1,000 others in the county, also reportedly effected by the regulation.

The problem is a classic example of a lack of coordination among federal and local welfare bureaucracies, according to Charles Vasaly, director of the legal services firm. Andrew Orlin, assistant chief of the Arlington County Division of Social Services, agreed, acknowledging that the law suit illustrated "an inequity."

The problem, officials pointed out, is that some low-income residents receive an average of $160 per month under a federal rent assistance program and that money does not count as income when it comes times to figure out food stamp benefits. But welfare monies distributed by the county under a similiar program is counted as income, reducing the amount of food stamps that some Arlington residents can receive.

What this means to Mildred S. Ruhe, an 86-year-old widow and one of those suing the department, is that she gets about $14 less per month in food stamps than she claims she should get. Legal Services attorneys claim that more than $10,000 a month in federal funds is being withheld from people who get benefits from an Arlington County rental housing program.

Charles Sabatino, a Legal Services attorney, has tried for more than a year to get the problem straightened out administratively. He testified last spring before a House Agriculture Committee hearing on the food stamp program and was told that the committee "specifically endorsed" a change in the regulation. But, he said, nothing was done and there was, according to Legal Services attorneys, no recourse but to sue the department.

USDA officials yesterday declined to comment on the dispute.