A coalition of 12 private social service organizations warned yesterday that District of Columbia budget cuts pending in Congress would harm "our most vulnerable and helpless children and elderly citizens."

Mayor Marion Barry, in a separate but related action, released a report by his budget director that said the city would lose 3,100 employes - 1 out of every 12 in its work force - if the budget is adopted in the form recommended May 23 by a House subcommittee.

Both warnings were issued on the eve of action scheduled for today by the full House Appropriation Committee on the city's budget for the 1980 fiscal year, which begins Oct. 1.

Whether the pleas will have any effect is an open question. The full committee rarely overturns recommendations from one of its subcommittee chairmen. In this instance, it is a new chairman, Rep. Charles Wilson (D-Tex.), presenting his first proposed city budget.

Wilson recommended that $78 million be cut from the city's proposed $1.35 billion operating budget, including $25 million that had been requested to pay for the city's growing cost of employe salaries.

The social service agencies, calling themselves the community Coalition on Financial Accountability, said in a statement distributed yesterday that most of the proposed cuts are "in direct contradiction to previous congressional mandates and to court orders" that require the city to provide services to its citizens.

In this category, the statement said, Wilson's subcommittee cut $290,100 and 20 positions slated for mental health services for public school children found to be emotionally disturbed, and for those too disturbed to attend regular classes.

"The most shocking cuts," the coalition said, total $1.1 million of $2.2 million that had been sought for improvements at Forest Haven, the city's home for the mentally retarded.

"It is hard to believe that these cuts were made by members of the same (subcommittee) that only two years ago deplored conditions at Forest Haven and ordered the city to make major improvements" to bring it into line with federal laws and court decisions, the statement continued.

Likewise, the coalition said, $6.2 million was cut from a proposed $8 million rise in public assistance payments to welfare clients.

Barry's budget director, Gladys Mack, said in her analysis that the reduction of the city's payroll by $25 million through attrition would mean a loss of 3,100 employes rather than the 1,700 estimated earlier.

"It must be pointed out that exacting savings through attrition produces extremely disproportionate reductions from program to program," Mack wrote. Entire small agencies might be wiped out, she said.

Among the big employe losses, Mack said, would be 710 from education, 329 from the police department, 99 from the fire department, 290 from the corrections department, 345 from human resources and 318 from environmental services. The D.C. City Council would lose 23 employes, or 27 percent of a staff already reduced by the termination of its federally subsidized CETA jobs program.