An illegal migrant on Long Island fatally butchered a beloved mom of three and another woman in separate savage attacks — before calling cops and calmly confessing he “killed somebody,’’ police said.

Rony Yahir Alvarenga Rivera, 22, of El Salvador — who entered the US as an unaccompanied 12-year-old in 2016 — knifed a 42-year-old coworker and mom to death at an Island Park Wendy’s around 12:30 a.m. Friday, authorities said.

The victim, identified by CBS-TV News as Ana Maria de Aguila Cordova, a mother of three from Long Beach, was taking out the trash when she was ambushed.

“I can’t stop crying,’’ a weeping female Wendy’s coworker told The Post on Sunday, pointing to the blood-stained spot on the sidewalk outside the fast-food restaurant where the hardworking mother was killed.

Three hours earlier, around 9 p.m. Thursday, Rivera had hacked his 32-year-old roommate to death at their Valley Stream apartment, according to Nassau County police.The tragic woman was identified by cops as Eddy Raquel Hernandez Castillo.

Rivera later went to his job at Wendy’s and killed de Aguila, cops said.

Both of the dead women suffered brutal stab wounds to the neck and torso.

Rivera then allegedly called the police and told them what he had done.

“While we were at the [Wendy’s] scene, we received information that an individual was stopped at 169 Atlantic Avenue in Lynbrook,” Detective Lt. George Darienzo said.

“He asked to see police and when police arrived, he said he killed somebody that night,” he said.

Nassau County Executive Bruce Blakeman said at a press briefing, “This didn’t have to happen.

“This is just an example of what happens when you have open borders.

“Why are we letting an unaccompanied minor into our country to run around, basically without any supervision whatsoever?” Blakeman said. “And then that individual grows up and automatically gets legal status in our country. This is a policy today that resulted in two women getting killed.”

A gofundme page said to be set up for de Aguila’s family described the victim as the married mother of an 8-year-old and 2-year-old on Long Island, with a 22-year-old son in Peru.

“To those who knew Ana Maria, she will be remembered and cherished as a vibrant spirit,” the page said.

“With her unwavering enthusiasm and love, she gained the admiration from her colleagues, friends, and family alike. This is thanks to a blend of warmth, dedication, and unwavering work ethic that she gave out to the world.”

Durianzo said police had first responded to a 911 call at the Wendy’s at 33 Austin Blvd. in North Long Beach on reports of a man with a knife.

Cops found Cordova’s body in front the restaurant, where she was pronounced dead, the officer said.

While at that scene, police then got word of a dead woman at 90 West Mideola Ave. in Valley Stream, which is Rivera’s apartment, he said.

“They live in the same household, so they rent bedrooms at that location, and there’s general spaces throughout the house that they share — living room, kitchen, bathroom,” Durianzo said of Rivera and the first dead victim.

“So they cohabitate, they’re acquaintances of one another,” he said. “As far as a relationship, I’m not aware of a specific domestic relationship between those two.”

Rivera was arrested outside a 7-Eleven in Lynbrook after he called the cops on himself.

Police did not reveal a possible motive for the vicious slayings and did not identify the two victims.

The suspect had no prior arrests or gang affiliation, police said.

He is charged with first- and second-degree murder, police said.

The county executive, the GOP frontrunner to challenge Gov. Kathy Hochul in November, has long been critical of the state’s sanctuary status as a haven for illegal migrants.

He said Nassau County’s landmark agreement to work with federal immigration has led to the deportation of more than 2,000 migrants with serious criminal records.

“I don’t know what the criminality would be here and throughout this region if those 2,000 individuals were still roaming around,” Blakeman said.

“We cannot go back to the failed policies that have created this environment.”