Here the repeated infinitival to was accidentally omitted by the 1837 typesetter. As discussed under Helaman 7:5, either reading is theoretically possible, so here we follow the earliest extant sources (𝓟 and the 1830 edition) and their reading with the repeated to. The 1908 RLDS edition restored the to to the RLDS text by reference to 𝓟, while the to was restored to the LDS text in 1920 (most likely by reference to the 1830 edition, not the 1908 RLDS edition). For a complete discussion of the repeated infinitival to, see under conjunctive repetition in volume 3.
Summary: Maintain the repeated to in 3 Nephi 1:14, the reading of 𝓟 and the 1830 edition: “to fulfill all things … and to do the will both of the Father and of the Son”.