Robbie Sherwood, spokesman for the U.S. Attorney's Office in Arizona, said two of the immigrants pleaded guilty to an immigration violation Tuesday and the third is expected to do so Wednesday. They will be deported to Mexico.
Sherwood said in a written statement that there's no evidence tying the three to the Dec. 14 shooting.
The three immigrants from Mexico were arrested near the Arizona border city of Nogales the day after Border Patrol Agent Brian Terry was shot.
Terry was waiting with other agents in a canyon 13 miles north of the border when a shootout with bandits erupted.
Terry was the only person killed in the attack. No other agents were injured.
Another suspect who was shot in the incident is being held on immigration charges and hasn't been charged in Terry's death.
The statement from the U.S. Attorney's Office doesn't say whether other suspects are in custody.
In another development, FBI spokesman Manuel Johnson said there's no evidence that friendly fire played a part in the shootout.