The first three people arrested during a sit-in demonstration in Boston, including a Gen Ed Ahf section man, were found guilty of trespassing yesterday. Judge Jerome Troy of the Dorchester Municipal Court imposed the maximum fine of $20 apiece.

Peter G. Filene, teaching fellow in General Education, Mrs. Betty Wise of Brookline, and Andrew R. Phillips, a student at Boston University, appealed the verdict and were released in $200 personal recognizance.

Filene and his two companions were arrested after they had sat in for six hours at the G. V. Wattendorf Real Estate Co. in Dorchester. The Congress of Racial Equality (CORE), which sponsored the sit-in and simultaneous picketing outside the Wattendorf offices, charges that the company discriminates on the basis of race.

According to Filene, CORE testers found evidence of discrimination on two occasions. Last Tuesday, Wattendorf agents allegedly showed a white woman several apartments after telling a Negro none were available. On Friday, Filene said, Wattendorf sales manager John Coleman agreed to show an apartment to the Negro, Mrs. Alma Williams, and then showed her one in a vandalized building in a Negro section.

Edward J. Barshak, attorney for the demonstrators, argued that they had been on the property legally protesting discrimination. He based his defense on the statutary definition of trespassing as being on private property "without right." Last month, the Massachusetts Commission against Discrimination ordered the real estate company to stop discriminatory practices.

Judge Troy rejected Barshak's argument, however, ruling that the question of discrimination was irrelevant to the merits of the case.

Lawyer Doubtful

Although Judge Troy's decision is being appealed to Superior Court, Barshak doubts it will be carried much further because "the facts in this instance are not ideal for a test case."

Barshak said yesterday that CORE's case was weakened by the fact that Mrs. Williams, the Negro whose application for an apartment was rejected, was not among the sit-in demonstrators. Also, the arrests took place after the Wattendorf office had closed.

If the CORE demonstrators finally win this or a similar case, it will give sit-in protests on private property covered by fair housing statutes a legal sanction.