She took first place on the podium after winning the women's SM14 200m individual medley on Saturday night.
Firth, from Seaforde in County Down, broke her own Paralympic record with a time of 2:19.55 to win the race.
Her golden hat trick came after she successfully defended her 100m backstroke title and won the 200m freestyle gold in Rio last week.
She also won a silver medal in the 100m breaststroke final, considered the weakest of her four events.
Firth said that, as the medley was her last event in Rio she wanted to make a splash.
"I wanted to do something special and I'm so happy with my performance."
Her record-breaking win saw her clinch victory ahead of her British teammate Jessica-Jane Applegate.
The British one-two topped off a very successful night in the pool for the ParalympicsGB team.
Oliver Hynd smashed the world record in the 200m individual medley SM8 to retain his Paralympic title.
Firth's teammate Hannah Russell won her second gold medal in the 50m freestyle S12.
"We're such a good team, Great Britain were really pushing each other on," Firth said.
"When we saw Ollie win his gold and Hannah win the gold, it just sort of pushes each other on," she added.
Firth, who is 20, competes in the S14 class for swimmers with an intellectual disability.
Northern Ireland swimmer Bethany Firth is celebrating her third gold medal at the Paralympics in Brazil.