The earliest text here reads “concerning things which is to come”. In his editing for the 1837 edition, Joseph Smith changed the singular is to the plural are, in accord with standard English. The same kind of editing occurred earlier in the text for another example of this phraseology:
For a general discussion regarding subject-verb disagreement in the original Book of Mormon text, see under 1 Nephi 4:4 (as well as under subject-verb agreement in volume 3).
A more significant variant in this passage involves the loss of the noun things from the phrase “concerning things which are to come”. This error occurred when the compositor for the 1841 British edition set the type for this passage. The two following LDS editions (in 1849 and 1852) retained the difficult shorter reading, “concerning which are to come”. Finally, the 1879 LDS edition restored the necessary things. Elsewhere in the text there are no examples of “concerning which”. Instead, there is always a noun phrase between concerning and which. We get the following statistics for those cases when the noun phrase is generic in meaning (the count includes the case of “concerning things which” here in 3 Nephi 15:7):
“concerning that which” 9 times
“concerning the things which” 7 times
“concerning things which” 2 times
“concerning all things which” 1 time
“concerning the thing which” 1 time
The other instance of “concerning things which” occurs in 2 Nephi 6:4: “and now behold I would speak unto you concerning things which are and which are to come”. The critical text will maintain the instance of “concerning things which” here in 3 Nephi 15:7 (as well as restore the original singular is to the relative clause).
Summary: Maintain the generic noun things in 3 Nephi 15:7, the reading of the earliest textual sources; consistent with usage elsewhere in the original text, restore the singular is in the phrase “concerning things which is to come”.