Here in 3 Nephi 27:28, the original manuscript undoubtedly read things (since both 𝓟 and the 1830 edition read in the plural), but such a reading in 𝓞 could be a mistake since the original text is followed by it in the following clause. The contradiction in number between things and it may have been one reason the editors for the 1920 LDS edition decided to remove the it. Another reason for removing the it was its redundancy. For discussion of other cases where a redundant it has been removed from the text, see under Alma 43:46. Such redundancy is, to be sure, fairly frequent in the original text.
Another possible emendation here in 3 Nephi 27:28 would have been to leave the redundancy of the it but change the plural things to the singular thing. One could argue, in fact, that Oliver Cowdery (the probable scribe here in 𝓞) accidentally added the plural s while writing down Joseph Smith’s dictation. See under 1 Nephi 15:11 for several examples where the scribes, including Oliver, mixed up the grammatical number for thing(s).
As explained under 1 Nephi 18:6, the Book of Mormon text has plenty of examples of “whatsoever thing(s)”, with 11 instances in the singular and 11 in the plural (in the original text). So either is theoretically possible here in 3 Nephi 27:28. In fact, there are three cases where it is used to refer to a preceding singular “whatsoever thing”:
But there is also one instance where it refers to a preceding plural “whatsoever things”; this instance of disagreement in number has never been removed from the text:
This example shows that the earliest reading here in 3 Nephi 27:28 is textually acceptable. The critical text will therefore restore the original it here even though it refers to a plural “whatsoever things”. More generally, we should note, the original Book of Mormon text allows for the singular it to refer to plural subjects. For some additional examples, see the list under Helaman 13:31. And as explained under Alma 43:46, the redundancy of the it is also textually acceptable.
Summary: Restore in 3 Nephi 27:28 the pronoun it in “it shall be given unto you” even though the it is redundant and refers to the earlier plural “whatsoever things”.