The 23-year-old's partially submerged body was found at around 8.25pm on the outskirts of Athlone, County Westmeath, the Garda said.
A Garda spokesman said: "The body of the man was discovered partially submerged in water at the River Shannon in an area known as Bogginfin.
"The area has been sealed off pending a full Garda technical examination."
A post-mortem on the body is due to be carried out later today.
The three arrested men are being held at police stations in Athlone and Mullingar.
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