Here the 1874 RLDS edition accidentally omitted the repeated preposition for. The 1908 RLDS edition restored the for to the RLDS text. In theory, either reading is possible. The critical text will follow the earliest reading, with the repeated for. For further examples where prepositions are repeated in conjuncts, see under conjunctive repetition in volume 3.
Summary: Maintain in Alma 36:16 the repeated preposition for in the conjunctive prepositional phrase “for three days and for three nights”.