Three busloads of students from the International School of Law in Arlington lobbied Virginia legislators today amid sings the proposal to merge the school with George Mason University has taken on new life.

Wearing "Urge to Merge" buttons, the 120 students pleaded their cause to General Assembly members who have been skeptical of the need for a third state law school and fearful of its costs.

However, Sen. Hunter B. Andrews (D-Hampton), influential chairman of the Senate Education Committee, said he is rethinking his previous opposition to establishing a law school at George mason.

"I have made no commitments to support it," Andrews said, "but it is a materially changed situation. They have a building. The school is operational. They have 145,000 square feet of space, which gives them a good deal of excess to lease out for income for the school. They will have to file a statement on the fiscal impact on the state and we will have to study it."

George Mason's uphill fight to win state approval of a law school was given new momentum last year when the George Mason University Foundation acquired the Arlington school a 3401 Fairfax Dr. and 11 acres of commercially zoned land around it.

Advocates of merger of the law school and Northern Virginia's state university in Fairfax City contend that it would cost the state less than$1 million a year for the next five years. After that, they say, tuition payments and income from the commercial property would enable the school to pay all its expenses.

Despite this forecast by George Mason supporters, the state Council of Higher Education recommended against the merger in November after receiving a mixed report on the proposal from a consultant and its own staff.

The council said the state should concentrate on raising faculty salaries and making other improvements at existing state law schools at the University of Virginia and College of William and Mary. Two private universities in Virginia, Washington and Lee and the University of Richmond, also have law schools.

Sen. Omer L. Hirst, who is retiring this year as Northern Virginia's senior legislatro has made extablishment of a state law school at George Mason a major legislative goal of his last year in the Assembly. The proposal also has been endorsed by Democratic Lt. Gov. Charles S. Robb and Republican Attorney General J. Marshall Coleman, considered most likely to be the major party candidates for governor in 1981.