According to Daily Mail, the toddler was sitting in a motel room in Albuquerque, New Mexico, with his mother, Monique Villescas, father, Justin Reynolds, and two-year-old sister on Saturday when the shooting occurred.
He reportedly reached into Miss Villescas's purse in a bid to reach her iPod, but unwittingly pulled out a gun instead. Seconds later, he fired one shot, striking his father in the right buttock.
The bullet then traveled out of Mr Reynolds's hip and struck his long-term girlfriend - who is eight months' pregnant - in the arm, police said. The pair were rushed to hospital with gunshot wounds.
Mr Reynolds has since been released. Miss Villescas's condition remains unknown. However, police said the expectant mother appeared to have been 'seriously' injured in the shooting.
The boy was able to remove a loaded handgun from his mother's purse, before shooting his father in the lower backside on Saturday.
The bullet then exited the man's body and struck the pregnant woman in the shoulder, according to local police.
Both parents were treated at a nearby hospital. The father has since been released, but the mother, who is eight months pregnant, was hospitalised.
She is believed to be in a stable condition, however the condition of her unborn baby is unknown.
Robb Robillard, who was staying at the motel when the shooting occurred, said that it was "scary" a three-year-old was the culprit.
"I came out of the room and saw police officers downstairs and I came down the stairs and saw a whole bunch of cop cars," he told KRQE News 13.
A two-year-old girl was found by police in the room, but she was not injured. The children have been placed in the custody of the Children Youth and Families Department for a minimum of 48 hours.
Local animal control officials were also called to remove two pit bulls that were discovered in the motel room.
Police said that the parents could face charges of criminal negligence for allowing a loaded gun to get into the possession of a child.
The District Attorney's office will be notified and will decide whether to press charges.
The case is strikingly similar to a shooting in December last year, when a woman, 29, was fatally shot by her two-year-old son in an Idaho Walmart, after he found a gun in his mother's handbag.
The American toddler shot his father and his pregnant mother with a single bullet after reaching into her purse for an iPod, but instead pulling out a handgun.
Seconds after grabbing the 9mm pistol, the 3-year-old boy unwittingly fired one shot, which hit his father in the buttock, passed out of his hip and then struck his mother in the shoulder.
Justin Reynolds and Monique Villescas were taken to hospital in Albuquerque, New Mexico. Both are expected to survive, but Villescas, who is eight months pregnant, was described as "seriously" injured. The shooting was the latest in which a young child fired a relation's unsecured weapon, either accidentally or while playing.
In Missouri last month, a 5-year-old shot dead his 9-month-old brother after finding his grandfather's loaded pistol in the family home, and in Idaho last December, a mother shopping at a Walmart store died after her 2-year-old son reached into her purse and accidentally fired her handgun.
An average of two children a week are shot dead in accidents in the US and in three quarters of the cases, the gun was fired by another child.
Their son and his 2-year-old sister, who were staying with their parents in a motel room when the shooting occurred, were taken into temporary care by the state's social services while an investigation was conducted.
"All of a sudden we heard a gun go off," Reynolds told KOB4 television news channel.
"The next minute I realised my girlfriend was bleeding. Then I sat down and realised I was shot too."
