Two 20-year-olds, from Havant and Waterlooville, and a 22-year-old from Cosham were detained overnight and are in custody.
The arrests come after Georgia Jones, 18, had a “45-minute fit” after “taking two pills” at the Mutiny Festival on Saturday.
Ms Jones and 20-year-old Tommy Cowan both died, as organisers warned of a “bad batch” of drugs at the annual two-day gathering.
One person was in a critical condition and 12 others were treated in hospital after the first night of event.
A spokeswoman for the hospital said not all the admissions were “necessarily drugs-related”.
Georgia Jones’ mother, Janine Milburn, posted a message on Facebook to stop others from “ending up the same”.
Ms Milburn wrote on Facebook: “Georgia died yesterday due to complications after taking two pills at Mutiny.
“If nothing else I hope what has happened to her will deter you from taking anything ever.
“The pills had caused her temperature to rise so high it made her fit for 45 minutes.”
Mr Cowan’s father said he had held his son’s hand in the hospital before he died, and that he was not alone.
“Young boy, comes to a concert, makes one wrong decision and has died,” said Damian Cowan, 43.
He described his son as “brilliant, loved football, full of life, energetic, sarcastic, a brilliant dad”. Tommy has a son of his own who will be two this year.
“It’s no good saying don’t do drugs,” said Mr Cowan. “All I can say is take this on board, look what’s happened…”
He said he had heard security at the festival this year was “really bad and lax” and called for far tougher checks on people entering the event.
Mutiny Festival organisers warned at around 9.30pm on Saturday, after the pair were taken ill, that it was “aware of a dangerous high-strength or bad batch substance on site” and that no drugs should be taken.
They also urged customers to “responsibly dispose” of any substances.
Craig David and Sean Paul were due to headline on Sunday before the rest of the event was cancelled.
The organisers said in a statement on Facebook: “The safety of our amazing customers has always been paramount to us and so to keep everyone safe and in respect to those who have passed, we have taken the decision not to open today.
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“As you can imagine, this decision was not taken lightly and was taken with the support of the local statutory authorities who we continue to work with.
“Enquiries are being made into the circumstances of what has happened, but we must reiterate our advice to all our customers to responsibly dispose of any substances.”