Police along the Eastern Seaboard were searching yesterday for a van plastered with religious slogans which has stopped at Hartford, Conn., and Harrisonburg, Va., to drop off children who are either dead or seriously ill with infectious diarrhea.

Authorities said they believed that the three women and the 10 believed remaining in the van on a cross country trip were members of a religious sect who were avoiding medical attention because of religious beliefs.

Two children belief who had been in the van were reported dead yesterday and athird was reported in critical condition at the University of Virginia Hospital in Charlottesville. All had been left at hospitals suffering from untreated diarrhea, an illness that can be fatal to infants.

The Associated Press quoted a brother-in-law of one of the women as saying that the group had "a radical view of the Christian faith" and had decided to quarantine themselves . . . . They are not interested in being treated."

Police initially believed the group was traveling from Hartford to Nashville, Tenn., but a person who answered a telephone at the home of a relative of one of the women in Nashville said the group had changed its ininerary.

Harrisonburg police said last night they believed they had received a report of a positive sighting of the van at noon yesterday in Rockingham, N.C., near the North Carolina South Carolina border. Bulletins for police to be on the lookout for the van were transmitted to police departments throughout the eastern and southern states. "There are no charges against these people. We just want to find them to treat the children," one Virginia official said yesterday.

The first child was reported to have been dead on arrival Saturday at a Hartford, Conn., hospital and a second was pronounced dead on arrival at a Harrisonburg, Va., hospital Tuesday, police said.

Police said both children and a third child, reported in critical condition at the University of Virginia Hospital in Charlottesville, suffered from untreated diarrhea.

Virginia epidemiologist Grayson Miller told the AP that the three children all suffered from dehydration caused by untreated diarrhea. "The kids are getting very ill and are getting to the doctor too late," he said.

Miller said the disease - not fully diagnosed but probably shigella, a bacteria, that causes dysentery - is "not a community menace" and is not a killer unless its victims are left untreated.

Police said a two-week-old oby, identified as Joseph Hoskins, was taken by ambulance to the Charlottesville hospital from Harrisonburg with a woman who left on Wednesday without saying where she was going or how she could be reached. Police said a 4 1/2-month-old girl, identified by the AP as Nickima D. Pitts, was dead when she arrived at the Rockingham Memorial Hospital.

A child, 6 1/2-month-old Sarah Holloway, was dead on arrival last Saturday at a hospital in Harford, police and hospital officials said.

The van also disappeared on Wednesday and he has not been seen since police said. They described it as a light green, late model vehicle with religious slogans printed on its sides in red paint.