The Arlington School Board voted last night to close three elementary schools because of declining enrollment. In the last five years enrollment in Arlington's public schools has dropped by more than 25 percent.

The board voted unanimously to close Custis and Fairlington elementary schools, both located in South Arlington, next June. Fort Myer Elementary School, located on land owned by the Department of the Army, will close in June 1980 when Arlington's lease with the Army is due to expire.

Students from the three schools will attend other schools nearby, which in some cases are less than half a mile from the schools that will be closed. Next fall Custis and Fairlington students will attend Abingdon Elementary. In September 1980 Fort Myer students will attend either Key, Long Branch or Henry elementaries.

Fewer than 20 parents and teachers from the affected schools attended last night's board meeting, which had been preceded by several public hearings.

In voting to close Custis, the board, at the urging of School Superintendent Larry Cuban, rejected a proposal by that school's PTA that students from Fort Myer be bused to Custis, a distance of about two miles.

"There are no compelling reasons for such a move," Cuban told the board, adding that Custis is too small to handle an influx of Fort Myer students.

"We need to consolidate schools for educational and financial purposes," agreed board member Torrill B. Floyd. "I can't see very many reasons why this consolidation shouldn't be successful."

For the past three years the school board has considered closing each of the three schools because enrollment had dropped below 230 students, the number at which an elementary school is automatically considered for closing. Those decisions were delayed, officials said, in order to see whether population in these areas would increase.