Jamieson, Fausset and Brown on 1 Chronicles 18:3

3. Hadarezer--or, "Hadadezer" (2Sa 8:3), which was probably the original form of the name, was derived from Hadad, a Syrian deity. It seems to have become the official and hereditary title of the rulers of that kingdom.

Zobah--Its situation is determined by the words "unto" or "towards Hamath," a little to the northeast of Damascus, and is supposed by some to be the same place as in earlier times was called Hobah (Ge 14:15). Previous to the rise of Damascus, Zobah was the capital of the kingdom which held supremacy among the petty states of Syria.

as he went to stablish his dominion by the river Euphrates--Some refer this to David, who was seeking to extend his possessions in one direction towards a point bordering on the Euphrates, in accordance with the promise (Ge 15:18; Nu 24:17). But others are of opinion that, as David's name is mentioned (1Ch 18:4), this reference is most applicable to Hadadezer.