Favourite Rollins, who owned the fastest personal best in the field, ran an almost flawless race from lane six to add Olympic gold to her 2013 world title by a couple of paces in 12.48 seconds.
Ali was clear in second place when she crossed the line to take silver in 12.59 but Castlin, who was fourth over the final hurdle, faced a nervous wait before discovering she had done enough in the dash to the finish to take bronze in 12.61.
The trio of friends shrieked in delight and jumped up and down on the track in an embrace when the scoreboard confirmed they had secured the first Olympic podium sweep in the women's sprint hurdles.
"I knew I got the gold, I just wanted to make sure my team mates medalled as well," Rollins told reporters.
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"It's like a sisterhood, Kristi and I train together and tonight we prayed together to give us the confidence we need to get us through these rounds and come out here and fulfil our dreams.
"I'm just so grateful we were able to accomplish this together."
Cindy Ofili, who was also born in the United States but has an English mother and elected to run for Britain, pushed Castlin right to the line with her best run of the year but lost out on a medal by two hundredths of a second in 12.63.
PARTY TIME
European champion Cindy Roleder of Germany was quickest out of the blocks but had to settle for fifth in 12.74, ahead of Pedrya Seymour of the Bahamas, who set a national record in the semis and was going well until she hit the penultimate barrier.
Ofili's sister Tiffany Porter, also American-born but racing for Britain, was seventh with Canada's Phylicia George finishing last.
Rollins's triumph put the United States top of the medals table in the athletics at the Rio Games with five gold medals and 19 in total, ahead of Kenya and Jamaica on both counts.
The depth of the U.S. in the event was illustrated by the fact the world record holder Kendra Harrison and 2008 Olympic champion Dawn Harper missed the Games after failing to make the cut at the U.S. trials.
"We are just so strong, anybody in that trial was capable of winning a medal," Rollins added. "We just wanted to do our best and our best would get us on the podium."
Harrison and Harper were not the only notable absentees with world champion Danielle Williams failing to get through the Jamaican trials and Australia's Olympic champion Sally Pearson forced to withdraw from the Games with an injury.
Castlin said the three Americans who won together were now going to celebrate together.
"We are gonna have a great time, probably go out," she said. "Maybe we'll got the beach, get some dancing going, have a good time.
"Every night we are together is a girls night out."
(Editing by John O'Brien)
