A fourth person aboard was also injured in the crash on Tuesday night.
Italian speedboat race officials said the boat was trying to set a record for the route from Monte Carlo to Venice.
They said the boat had begun the challenge on Monday morning and was allowed one fuel step.
At least one of those killed was Italian, as was the injured man.
Italian state radio said the speedboat was capable of speeds up to 80mph.
The cause of the crash is under investigation.