Each of the brothers was already established in one kingdom - Lothar in Italy, Louis the German in Bavaria, and Charles the Bald in Aquitaine. Lothar? receieved the central portion of the empire - what later became the Low Countries, Lorraine, Alsace, Burgundy, Provence, and Italy - and the imperial title as an honor without more than nominal overlordship. [Louis the German]? receieved the eastern portion, much of what later became Germany. [Charles the Bald]? received the western portion, much of what later became France.
Though often presented as the beginning of a devolution or dissolution of Charlemagne's unitary empire, it in fact reflected the continued adherence to the Frankish idea of a partible or divisible inheritence rather than primogeniture.