The 1837 edition removed the adverbial ending -ly from sufficiently in favor of the adjective form sufficient. The assumption is that sufficient(ly) refers to some attribute of the pits; thus we expect an adjective after pits (as if the sentence read “they could not dig pits big or deep enough to hold them”). Another possibility (theoretical, at least) is that sufficient(ly) refers to the digging itself, not the size of the pits; in other words, they could not dig enough pits to hold them. This interpretation would imply that there were so many disciples that they couldn’t dig enough pits. Of course, this is clearly the wrong interpretation.
In fact, the original adverb form sufficiently does not actually refer to some particular characteristic of the pits that failed to hold the disciples. Instead, the meaning of sufficiently here in 3 Nephi 28:20 is ‘competently enough’. The Oxford English Dictionary indicates that in Early Modern English the adjective sufficient also had the meaning ‘competent, capable, able’, now obsolete, with citations listed under definition 3a dating from late Middle English into the early 1800s; examples where sufficient is complemented by an infinitival clause headed by to (listed under definition 3b) date from late Middle English to Early Modern English and include this example from Paradise Lost (book 3, lines 98–102):
Here Milton is referring to the agency of man and is stating that man was made capable of either standing or falling.
Another striking use of sufficient with the archaic meaning ‘competent enough’ is found later on in the Book of Mormon text:
In other words, Mormon has observed these conditions ever since he was capable or competent enough to do so. The word sufficient is not specifically referring to him being ‘old enough’ or ‘big enough’, although the question here is definitely one of maturation.
The critical text will therefore accept the adverbial form sufficiently here in 3 Nephi 28:20, with the understanding that it means ‘competently enough’; in other words, no pit was capable of holding Jesus’s disciples.
Summary: Restore in 3 Nephi 28:20 the adverbial form sufficiently; here the use of the word sufficiently is an archaic one, dating apparently from Early Modern English, that means ‘competently enough’; in other words, they could not dig pits that were capable of holding the disciples; the adjective form sufficient has the same archaic meaning of ‘competent enough’ in Mormon 2:18.