The printer’s manuscript has the plural instructions, while the 1830 edition has the singular instruction. The 1908 RLDS edition restored the plural reading to the RLDS text, while the LDS text has maintained the 1830 reading.
Usage elsewhere in the text argues for the plural instructions. There are a couple occurrences of the singular instruction, but these refer to general instruction and not to specific instructions:
When the text refers to someone’s specific instructions on how to do something, we get the plural instructions:
We note that the Alma 49:8 example, like the one here in 3 Nephi 3:26, has the phraseology “after the manner of someone’s instructions”. Even the example from Ether 2:16 has the nearby phrase “after the manner”. Thus internal evidence suggests that the plural reading in the printer’s manuscript (“after the manner of his instructions”) is the correct reading in 3 Nephi 3:26.
In Alma 49:8 (listed above), Oliver Cowdery initially wrote the plural instructions in 𝓟 as instruction, which he later corrected to the plural by inserting the plural s inline; 𝓞 is extant in that case and reads in the plural. This example shows Oliver miswriting the correct plural as a singular, the opposite of what we need here in 3 Nephi 3:26. It appears that the 1830 typesetter made that mistake when he changed an original plural to the singular in this passage.
Summary: Restore the plural instructions, the reading in 𝓟, for 3 Nephi 3:26 since this is what we get elsewhere in the text when the reference is to the specific directions from someone.