The Montgomery County Board of Education, seeking economics to prevent a property tax increase, voted last night to close three junior high schools in June and make other budget cuts.

The vote to close the schools -- to effect a saving of almost $1 million -- came as a surprise, since such decisions have been deferred pending completion of a master facilities plan, due in March or April. The schools to be closed will be designed this spring.

The actions came as the board completed its spending plan for the 1981-82 fiscal year by cutting more that $2 million from Superintendent Edward Andrews' original budget proposal. The budget total finally agreed on -- $336.6 million -- will be requested from the county executive and County Council, which told the board last fall it could fund a $335.4 million school budget without raising property taxes.

Last year the board requested $314 million and got $309.1 million, with the council cutting funds for fringe benefit for retirees and lowering average starting salaries for teachers.

School board members feel that because of inflationary increases in the value of property, the proposed budget will not require an increase in the property tax rate.

Although the board made other budget cuts -- among them $1 million from a fund for accrued liabilities and pensions -- the school closing decision was the most controversial.

Board member Marian L. Greenblatt proposed at first to close 10 elementary schools and three junior highs for a savings of $1.9 million, but lacked support from other members. she then made the successful motion to close only the junior highs.

"Is this board ready to close schools?" Greenblatt asked. "We cannot let the County Council do our dirty work for us."

"This is a prime example of doing policy in the budget," said member Elizabeth Spencer. "There is no sense in doing a master plan of facilities if we adopt this motion."

As approved, the budget calls for virtually no program improvements or expansions. Teacher will get 9.5 percent cost-of-living raises negotiated last year as part of a two-year constract.

County Executive Charles W. Gilchrist and Council President Ruth Spector sent a letter to the board yesterday saying that the amount allocated for utilities was inadequate and that underbudgeting meant the school system could either cut back school programs during the year or request supplemental funding later. They warned that they would not support any supplemental funding request. The board voted to increase spending for utilities by $125.000 well under the half-million dollar estimated short-fall.