"one-third of the army ... another third of the army." A "third" is one part out of three equal parts. (See: [[rc://en/ta/man/jit/translate-fraction]])
This means that he will go out with them to battle. This can be stated clearly. Alternate translation: "I myself will go with you to battle" or "I personally will go with you into battle" (See: [[rc://en/ta/man/jit/figs-explicit]])