As discussed nearby under 3 Nephi 2:12, Oliver Cowdery sometimes wrote their in place of a different determiner in the environment of their. Here in 3 Nephi 4:24, Oliver initially wrote their armies in the printer’s manuscript because of the their in the preceding and the following “the way of their retreat”. Oliver later caught his error, crossing out the their and supralinearly writing the correct his; the correction may have occurred when Oliver proofed 𝓟 against 𝓞 (the correction is written with somewhat heavier ink flow). The his occurs in the 1830 edition, which is also a firsthand copy of 𝓞 for this part of the text, so 𝓞 must have read his armies. And earlier in the verse we have an instance of the correct his armies (“he did send out his armies in the nighttime”).
Summary: Maintain in 3 Nephi 4:24 the determiner his for armies, the 1830 reading and the corrected reading in 𝓟.