Omar Rashaad Bey, 33, pleaded guilty in federal court in Philadelphia to three counts of engaging in illicit sexual contact in foreign places. He is scheduled to be sentenced in January and faces as many as 30 years in prison on each count.
Related charges of traveling for the purpose of engaging in illicit sexual conduct were dropped.
Bey accepted responsibility for his actions, but his lawyer, Fortunato Perri Jr., said Bey had the permission of the girls' families to engage them in relationships.
"The families acquiesced to these Islamic marriages," he said.
Perri said he plans to raise the issue of finacial support during sentencing in a bid for leniency.
Bey was first charged in January after police interviewed one of the girls, a U.S. citizen who caught the attention of authorities when she went to the U.S. Embassy in Cairo to apply for a passport for her 1-year-old son in 2008.
According to a grand jury indictment handed down in March, the girl, a Philadelphia resident, and Bey entered into "a union purporting to be an Islamic marriage" by way of an Internet chat in 2006, and the two traveled together to Cairo, where she later became pregnant and gave birth when she was 15.
Bey also maintained a sexual relationship with two sisters during that same time period, one of whom now has two children by him, prosecutors said.
According to a plea memo, Bey had a similar arrangement with the younger sister, which began in Egypt when the girl was 14. The girl gave birth to a boy in 2006, when she was 15. Bey fathered a second child with the girl when she was 17.
Her older sister also had a sexual relationship with Bey, starting around 2004 when the girl was under 16, authorities said.
According to the Philadelphia Daily News, during Wednesday's hearing Bey told U.S. District Judge Jan DuBois he was currently unmarried and had 12 children ranging in age from 6 months to 15 years.