Three Brothers
The Three Brothers were King Duff, King Kenneth II, and Prince Malcolm. The sons of King Maol Chalvim I.
History
The period in which each of the three brothers lived lasted for only thirteen years, from the time Malcolm was born at Edinburgh Castle in 954, to the time King Duff was the first brother to die at the Battle of Gaine in 967. ("Once Upon A Time There Were Three Brothers...")
Prince Malcolm would be the second brother to die, occurring at some point between 984 and 994 (from causes as yet unknown), with the remaining brother, King Kenneth II, alive until his murder in 995. ("Avalon" Part One) [1]
When Kenneth III, son of Duff, led his armies against King Constantine III in 997, his banner was that of three swords, representing the three brothers. ("Phoenix")
Real World Background
Kings Duff and Kenneth II were indeed the sons of the historical King Maol Chalvim I, but Prince Malcolm was a fictional addition to the series. [2]