This is found in Geoffrey of Monmouth's Historia Regum Brittaniae (History of the Kings of Britain. See Other Works). Geoffrey uses Arthur's battle at Loch Lomond as an excuse to bring in three watery Wonders from Nennius (Lake Lumonoy, Fount Guur Helic, and Linn Liuan). He includes information not in the versions of the "Nennius" Historia that we have (at least those published), and it seems likely that he had a different manuscript. You can read the relevant sections of the Historia Regum Brittaniae here. The fact that there are three of these may suggest a traditional Welsh triad (see, for other examples, Bromwich*) on "the three marvellous lakes", though this is not apparent in the "Nennius" Historia.