Three former city policemen, who had escaped punishment in a state court trial, were given one-year federal prison terms yesterday for violating the civil rights to a Mexican-American prisoner who drowned here after they had beaten him and one allegedly threw him into a bayou.

The sentences, which could have ranged up to life, brought relief to the officers' supporters, but bitterness to the Mexican-Americans who had rallied in the cause of the victim, a hapless Vietnam war veteran named Joe Campos Torres.

Federal District Court Judge Ross N. Sterling explained the sentence by saying that the three men did not intend for Torres to die and that the three will never again find themselves in a position to repeat their crimes.

"That is aw-right," said a smiling Houston policeman, J. W. Zahn, to a fellow officer after the 23-minute sentencing. "That ain't bad at all. They won't be there in prison but six months."

In fact, with time for good behavior, the three, Terry Wayne Denson, Stephen Orlando and Joseph James Janish, should each serve about nine months. They would then face five years of probation. The one-year term is little more than federal judges here usually hand down for immigration violations, according to the Administrative Office of U.S. Courts in Washington.

"I thought the [Federal] government was going to take care of everything," said Margaret Torres, the victim's mother. "It's just a slap in the face. It's just getting away with murder."

"I got more than that when I broke probation as a kid," said an angry brother, Gilbert Torres. "I don't think it was right. It wasn't right at all."

The federal civil rights charges were pressed under a new Carter administration policy calling for such prosecutions when state investigations or trials end in a manner that federal prosecutors consider unsatisfactory.That policy reversed a 20-year-old Justice Department practice of not acting where states already had.

That resulted in a life sentence for a former Castroville, Tex., police chief who earlier had received a 10-year sentence on a state charge of gunning down Rich Ricardo Morales, a Mexican-American he had driven to a remote area.

The Torres case had become a rallying point among Mexican-Americans in Texas and the Southwest, where 15 of them have met death at the hands of lawmen in the past few years. Leaders in the Mexican-American community, citing alleged prejudice by police, prosecutors, and juries, claim that these deaths go virtually uninvestigated, or unpunished when prosecution does result.

Orlando and Denson previously had been convicted of a monor homicide charge by a state jury that sentenced them to two years and then suspended that.

Mexican-American leaders had hoped that the Castroville case and a stiff sentence yesterday would deter law officers from alleged mistreatment of Mexican-Americans. But Ruben Bonilla of the League of United Latin American citizens said yesterday: "The progress Mexican-Americans had made in seeking the administration of justice has been lost."

Sterling said in imposing sentence that he felt "a long period of confinement would have little impact upon the operation of the Houston police department [where] I conceive the heart of the problem to lie." The department has come under increasing criticism for what some people see as repeated excesses in the use of force.

Federal prosecutors are now investigating the fatal shooting by Houston police of a 17-year-old Louisiana youth last year. They have learned that the gun the youth allegedly pointed at officers had been confiscated by police in 1964 after a suicide and had been listed as "destroyed" in police records.