FBI agents yesterday served subpoenas on three Boston antiwar activists, instructing them to testify before a Harrisburg grand jury which indicted the Rev. Philip F. Berrigan and five other persons for an alleged bombing-kidnap conspiracy last January.

The subpoenas-delivered to Paul Couming, Claudette Piper and Ann Walsh-were among 20 such orders which federal agents served in New York City, Philadelphia, Baltimore, and Lewisburg, Pa.

The Harrisburg grand jury has been investigating federal charges of a conspiracy since last Dec. 1, four days after FBI director J. Edgar Hoover claimed in widely publicized Senate testimony that a plot had been made "to blow up electrical conduits and steampipes" in Washington and "kidnap a highly placed government official [later said to be Henry A. Kissinger]."

Others

The others indicted with Berrigan in the case are Eqbal Ahmad-a radical antiwar scholar-Sister Elizabeth McAlister, Father Neil McLaughlin, Anthony Schoblic-a former priest-and Father Joseph Wenderoth. They have all denied the charges against them.

In a joint statement issued yesterday, the three Boston activists said, "We consider it significant that these subpoenas were issued just prior to the antiwar movement's spring offensive." Their statement left open the question of whether they would obey the subpoenas. Several persons served with similar subpoenas have already refused to testify.