As many as 3,000 District of Columbia families could lose their entitlement to welfare payments -- and be cut off from assistance checks in January -- because they have not properly filled out a complicated new form on which they are supposed to report earned income and other information that might affect their eligibility for aid.

The families have until the close of business on Monday to complete the welfare certification form, which is mandated by new Reagan administration guidelines. If they do not, they automatically lose their certification for Aid to Families with Dependent Children (AFDC) support payments, the nation's basic welfare program.

Lee Partridge, a special assistant to D.C. Department of Human Services Director James E. Buford, said the city, in gradually applying new requirements that AFDC recipients report their incomes every month-- instead of every half year -- asked 5,900 of the 28,000 AFDC families here to complete the form and return it by Dec. 10.

But as of Tuesday, Partridge said, nearly 2,000 families had not returned the form. In addition, she said, about 1,000 families that did return the form filled out parts of it incorrectly or left parts blank, making a total of 3,000 families whose benefits are jeopardized. If the forms are not complete by Monday, she said, the families will be taken off the AFDC rolls.

If they lose eligibility, the families will receive no support checks for January, Partridge said. They can subsequently reapply and their names will be put back on the rolls within about 48 hours, she said, provided that they have all the required documentation. In that case, it would take up to four days for the city to issue a retroactive check, Partridge said.

She said that eventually all the city's AFDC recipients will be required to use the new form to report income each month. She said that the city decided to phase in the new reporting requirements and began with the 5,900 families deemed to stand the most chance of having their benefits affected by new rules.

These include families in which there is at least one working member or in which there is a work history. The Reagan administration's welfare policies have been attacked for having adverse impact on the so-called "working poor," and many of these families would fit into that category.

The lengthy form asks aid recipients to list all the people in their households to determine if the current level of support is at the correct level. It asks for detailed information on before-tax income from a job, on other income and on the cost of health care for children or disabled persons, which the household must bear.

The form also asks for information about the recipient's assets: whether he or she owns a car or real estate, has a checking or savings account, whether the household has cash on hand, the amount of rent or mortgage that must be paid each month and other data.

The Rev. Jack Woodard, rector of St. Stephen and the Incarnation Church at 16th and Newton streets NW, has been sponsoring classes for recipients on how to fill out the new form, which he said "would make the Internal Revenue Service Form 1040 look simple."

He pointed to a section of the form headed "Other Income," which asks recipients to attach verification of any additional benefits they receive. The form asks that the recipient itemize, among other things, "accrued statutory benefits."

In another section, the form states, "The Department must consider as income any advanced Earned Income Tax Credit (EITC) Payments to which you may be entitled. If you are not receiving the EITC, you should apply for it immediately. If your employer refuses to pay the advanced EITC, submit proof of this from your employer."

Partridge said that changes will be made in the form to simplify it, adding that it was based on one developed in Massachusetts. She said, moreover, that the failure of one- third of the recipient households to respond is not unprecedented. The State of Delaware, she said, experienced a similarly small reponse when it tested its first forms under the new guidelines.

She said that families that have not properly completed the forms have been notified that they must get in touch with the Department of Human Services and arrange to complete the forms by the close of business on Monday.