Both the 1830 edition and the printer’s manuscript have the adverb already, not the adjective ready. The 1837 edition replaced already with ready, probably accidentally. The 1908 RLDS edition restored the already to the RLDS text (apparently by reference to 𝓟). The LDS text has retained the secondary ready. There are no other examples in the Book of Mormon textual history where already and ready have ever been mixed up.
Cases where the adjective ready is asyndetically conjoined with another adjective do not otherwise occur in the Book of Mormon text (as in “is ready / open to receive them”). On the other hand, there are examples where already is followed by a past-participial verb form that acts adjectivally (much like “is already open” here in 3 Nephi 18:13):
On the other hand, when ready is used as a predicate adjective, we expect it to be followed by an infinitive clause headed by the infinitive marker to:
Thus the original text for 3 Nephi 18:13 (“is already open to receive them”) is what we expect, but the secondary “is ready / open to receive them” is definitely a unique construction for the Book of Mormon.
Finally, the passage here in 3 Nephi 18:13 parallels one in 3 Nephi 11:40: “and the gates of hell standeth open to receive such”. Since one can refer to the gates of hell as already standing open, the original reading in 3 Nephi 18:13 (“and the gates of hell is already open to receive them”) is perfectly fine and will be restored in the critical text.
Summary: In accord with the earliest textual sources, restore the original already in 3 Nephi 18:13: “and the gates of hell is already open to receive them”; this reading is indirectly supported by usage elsewhere in the text.