Corporation DB operates three different lines of business. Can the corporation elect a different overall method of accounting for each line or must the corporation adopt one overall method

Yes, Corporation DB can choose a different overall method of accounting for every business unit that it possesses. As long as the business units operate separately, the corporation can use any permitted (legal) accounting method for them. For example, an American corporation uses GAAP in their domestic operations, but must use IFRS on all their foreign operations. (because GAAP only applies to the US).