The body of the youngster was discovered in the Ogmore River near Pandy Park in Bridgend after police were called to a report of a missing child in the early hours of Saturday morning.
He was named by South Wales Police on Monday.
A 39-year-old man, a 30-year-old woman and a 13-year-old boy, all from the Bridgend area, are being held in connection with the death, South Wales Police said on Sunday evening.
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Officers are not looking for anyone else, the force added.
An investigation is continuing into the circumstances of the incident, with police calling for witnesses who were in the area at 5.45am on Saturday to come forward with information about how the child ended up in the water.
Family friends paid tribute on social media to the “kind, funny, polite, handsome & clever” boy.