Unidentified gunmen opened fire while a boxing tournament weigh-in was going on at the Regency Hotel, around two miles north of the city centre.
Men brandishing AK47s and wearing 'police-style uniforms', entered the lobby and started shooting at random, witnesses said.
Witnesses, including several children were forced to flee the scene as shots were fired.
Police said all three victims were in their 20s or 30s. One was pronounced dead at the scene.
The two others were taken to separate Dublin hospitals with suspected gunshot wounds.
Gardai said they were called along with paramedics to the scene at around 2.30pm on Friday afternoon.
The two other victims have been rushed to The Mater and Beaumont hospitals with what are suspected to be gunshot wounds.
Boxers were weighing-in ahead of Saturday's 'Clash of the Clans' bout at the city's National Stadium.
Fighters on the bill include Dublin boxer Jamie Kavanagh, who will fight for the WBO European lightweight title against Portuguese Antonio Joao Bento.
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