In the scenario as it stands the Carthaginians break on the usual army break rules - this means it will probably go before the Carthaginian 3rd line is engaged to any great extent (at least against the AI, I think a Human player would spend time getting these good troops into the fray as quickly as they could). However, in the actual battle Hannibal was only finally defeated after all 3 of his lines were beaten - and the 3rd line put up a very good fight, only caving in when charged in the rear by the Roman cavalry.
Perhaps it could be represented by a short 2 game campaign. First battle is against the Carthaginians except all the veteran troops of the 3rd line. If the Romans win this what is left of their army carries over into the 2nd battle to fight the Carthaginian 3rd line (which may get some 1st and 2nd line remnants added). This would also allow for the Romans to undertake the historical reforming of their army.
Obviously if the Romans manage to lose the first battle there is no second battle
