Three members of the striking air traffic controllers' union in Northern Virginia were indicted yesterday by a federal grand jury on charges of participating in an illegal work stoppage against the government.

Kenneth Conklin, a Washington lawyer representing the three men, promptly labeled the action an indication of the "vindictiveness" with which the Reagan administration has obtained roughly 77 such indictments against strikers nationwide.

Conklin said the move appeared to run counter to attempts to give the two sides time to work out their differences amicably by delaying the date for final hearings in civil contempt proceedings against many of the union members.

The hearing date for the civil cases in Alexandria, where yesterday's criminal indictments were returned, has been postponed from Aug. 24 until later this month. The illegal striking charge is a felony with a possible jail term of one year and one day.

Named in the latest indictments were John F. Thornton, head of the Professional Air Traffic Controllers' Organization at National Airport; Thomas S. Galloway, head of the local at the Air Traffic Control Center in Leesburg; and William B. Lombardi, also a union member at Leesburg.

All three stated publicly when the strike began Aug. 3 that they would refuse to return to work unless working conditions at the facilities improved.

Galloway and Thornton subsequently were among five PATCO members fined by District Judge Oren R. Lewis in Alexandria for contempt of court for refusing to go back to their jobs despite a court order.

Lewis is scheduled to hear final arguments regarding the fines when the two sides meet in court later this month.