The Fairfax County Board of Supervisors yesterday approved preliminary plans to open three walk-in shelters for the homeless, joining other Washington area suburban jurisdictions in efforts to combat what has long been considered primarily an urban problem.

The shelters would house 50 homeless persons each at an estimated cost to the county of about $350,000 under the plan tentatively accepted by the board. The plan also calls for a 12-bed residential facility that would provide services for homeless persons with mental deficiences.

The proposal grew out of a controversy last winter involving Fairfax churches that provided housing for the homeless in what some county officials considered a violation of county ordinances. Area churches will be allowed this winter to provide shelter for the homeless while the county reconsiders those laws.

Although the board voted unanimously to endorse the shelters, which would be operated jointly by the county and religious groups, some supervisors expressed reservations.

"I'm not sure we want to become a magnet for the homeless," said Chairman John F. Herrity, a Republican.

Deputy County Executive for Human Services Verdia Haywood hailed the board's action as a a major step forward in "filling a gap in emergency housing" in the county.

"The homeless in Fairfax County are hidden from public view, unlike those individuals easily observed sleeping on grates and park benches in large urban centers such as D.C.," according to a report filed by the Shelter Coordinating Committee, composed of area volunteer and church groups.

The Rev. Vin A. Harwell, a member of the committee, said the board's action is "the beginning of Fairfax County's being able to face up seriously to the problem of homeless in the county."

County officials said they hope to open the three shelters by next fall. This winter, however, nine county churches will rotate responsibility for providing shelter for the homeless, Harwell said.

He said the first two shelters will open Monday. Harwell said the shelters were open for about 94 nights last year, with as many as 40 persons seeking shelter each night.

County officials said they will begin searching for locations for the shelters and begin coordinating plans for volunteers and county-paid employes to staff the facilities. Haywood said facilities will be provided in the northern, southern and central areas of the county. Possible locations include the Rte. 1 corridor and the Baileys Crossroads area, Haywood said.