Matthew Musolino III, 18, the third and final defendant in the Fort Hunt High School arson case, was sentenced yesterday to serve one year in jail, perform 3,000 hours of community service and make $10,000 in financial restitution to the county.

Fairfax County Circuit Judge Lewis H. Griffith, who called the December 1978 arson "a totally irresponsible, ignorant and stupid act," nevertheless suspended six mmonths and one day of the jail term. With good behavior, Musolino could be paroled after 119 days in custody.

The sentence was identical to that given two other teen-aged defendants in the case, both of whom were freed July 20 after serving 119 days in Fairfax County jail. County prosecutor Robert F. Horan Jr. recently criticized aVirginia statute that permitted the youths' early release as "really crazy" and "terribly misleading."

Griffith and assistant Fairfax prosecutor Steven A. Merril yesterday acknowledged that Musolino had surrendred to police after the fire and had cooperated with prosecutors. He was allowed to plead guilty earlier to a lesser, misdemeanor charge in exchange for his assistance.

"I would argue for 12 months in jail," Merril said, "but in light of what has happened to the other defendants, I can hardly say that anymore."

Robert P. Smithwick, 18, who threw the gasoline bottle that started the fire and gutted the school, pleaded guilty to a felony charge of malicious burning.The third defendant, Timothy Greer, 19, was convicted of the same charge in a jury trial.

Musolino, clad in a dark blue suit, sat with his head bowed and his hands clasped during much of the proceeding. He seemed oblivious to the crowded courtroom and the spectators lined up along both walls.

Under the same order as Smithwick and Greer, Musolino, who was convicted of destroying public property, will be given 10 years to pay the $10,000 from his own employment. As with his codefendants, it will take Musolino slightly over two years to complete the 3,000 hours of community service.

"I consider the three of you equally culpable for this crime," Griffith said before imposing sentence.

"This was a totally irresponsible, ignorant and stupid act. And had there been an injury or death as a result of this fire, you could have received today a possible life sentence."

Griffith commended Musolino for "admitting what happened and coming forward with the evidence," though he added that he did not completely agree with Musolino's version of the events that led up to the fire.

"With the amount of alcohol each of you consumed that night, I'm not certain you really know what happened. I prefer to think that the truth lay somewhere in a composite of the versions each of you gave," Griffith said.

I don't believe any of you intended to burn the whole school, but you intended to cause considerable damage. You, Matthew, knew what you were doing. I believe you broke the windows so that Smithwick could throw the bottle inside."

Griffith added that the sentence was not influenced by Musolino's four court appearances since last December's fire. Those appearances were on unrelated charges.

Musolino was convicted and sentenced on Monday to 360 days in jail on an assualt charge stemming from a June 24 incident involving a Fairfax County teen-ager. The conviction will be appealed in September in circuit court. Musolino was released the day following the incident on a $2,000 appeal bond.

Defense attorney Frank W. Dunham Jr., calling the arson case "the toughest I have faced in my years as a lwayer," called a number of character witnesses in an effort to persuade Griffith that Musolino's home life was a major cause of his transgressions against the law.

They included Air Force Col. William H. Roberts and his wife, Lorie, who qave allowed Musolino to stay at their home while on bond in the arson case. Musolino has dated the Roberts' daughter, Jennifer, for three years.

"Nat was exposed to violance. His home and his father's gas station were broken into. He saw two people killed at that gas station. He doesn't have a mother. When he was at our home he was different and we love him," Mrs. Roberts said.

Dunham argued that Musolino, who received a football scholarship to Louisiana Tech University before the arson incident, should have been incarcerated only until the school term ws about to start. He also contended that Musolino should serve his restitution after college.

But Griffith strongly disagree.

Dunham later verified that Musolino would enroll at an alcoholic counseling program immediately after his release from the Fairfax jail. Alexandria Hospital has already agreed to allow Musolino to perform his community service at that facility.

In words spoken so softly that Griffith leaned forward to listen, Musolino said after hearing his sentence, "it just happened. . . . I don't look at the community service as a punishment. . . . It's a way to show I do care. . . . I can't pay it all back, but I can get back a little pride." CAPTION: Picture, MATTHEW MUSOLINO III . . . identical sentence