Multi-Mind Mayhem: Isekai Tensei Soudouki

Bard Cornelius: the son of a nobleman of Mauricia, an empire located in a parallel universe. But Bard is no ordinary boy―he's got three souls packed into one body! Aside from his own consciousness, he's got Oka Sanai, a miserly samurai, and Masaharu Oka, a high school otaku who loves animal ears. With his extra knowledge of military tactics and business acumen, Bard's ready to cheat his way to the top!

(Source: One Peace Books)