It is unclear whether or not the three days is symbolic here. There is no real clear evidence that the resurrection theme of three days would have been known. Nevertheless, Jonah’s three days in the belly of the beast would also serve to underline the salvific nature of the three days here (see Jonah 1:17). Particularly in a story told for the generalized purpose of modeling a conversion, the three days becomes highly symbolic for those of us who recognize the reference to Christ’s resurrection.
Symbolically, Alma dies and is resurrected. In Pauline terms, he has laid down corruption and arisen in incorruption. In other gospel analogies, he has undergone a baptismal event (symbolically, of course).