The teens, including one from Illinois, were sent to the home by their parents to get them away from problems with drugs or friends, police Sgt. Shauna Greening said, reported CBS affiliate KUTV-TV.
The director, who was tied up with power cords Tuesday, eventually broke free and called police, Greening said. She also freed another pregnant teen tied up in the attack, she said.
Authorities believed the teens — two 15-year-olds and one 16-year-old — left the state and a search was under way. Police said the teens also stole the director's purse, checkbook, credit cards, cell phone and video camera.
The vehicle's Utah license plate number is: 128VTX, police said.
The motive wasn't clear. Parents of the girls, who are from Illinois, California and Texas, were notified, police said.
The New Hope home for struggling pregnant teens is in Utah County, about 30 miles south of Salt Lake City. Pregnant teens are typically sent there by parents to get away from problems with drugs or boyfriends, and they attend class and learn about prenatal care, childbirth and adoption while staying at the home, according to police.
A call by The Associated Press to a phone number listed for New Hope went unanswered Thursday.
But the owner, Spencer Moody, tearfully told a Salt Lake City TV station that he would close the rural home. He said about two dozen girls had given birth after living at New Hope.
Moody said he believed the girls were scared and confused when they attacked the director. "When they left, they told her, you are a good person, we just have to do this."
There is no excuse for what they did but they made a decision they really didn't think through I think," Moody said.
Greening would not release the teens' hometowns or the name of the director, but said the director "never had any indication that anything like this was going to happen. They were all sitting around doing homework before the attack occurred."