David Chudleigh (personal communication, 13 October 2004) suggests that there needs to be commas around the relative clause “which I am commanded of the Father to speak unto you” so that the sentence-final prepositional phrase “at this time” will refer to the earlier clause “ye cannot understand all my words”. One difficulty with this proposal is that the relative clause appears to be restrictive, so placing a comma before which might be problematic. The placement of some kind of clausal break before “at this time” appears to be correct, for if the Father had commanded Jesus to speak those words at that time, then Jesus would have. Instead, he will postpone those words for a later visit when the people will be able to understand.
What we have here is an example of a displaced prepositional phrase. The original Book of Mormon text has a good many instances of such prepositional phrases, and in virtually every instance they have been retained in the current text (as here in 3 Nephi 17:2). Here is a similar example involving an intervening restrictive relative clause; in this instance, editors have placed a comma after the relative clause (but not before it):
There have been some other ways of dealing with displaced prepositional phrases. In the following example from the original text, the phrase “at this time” is positioned in front of the relative clause that it properly belongs to:
Or equivalently, “because of the great destruction which doth await you at this time”. In this instance, the editors for the 1920 LDS edition moved the prepositional phrase “at this time” after the which, giving “which at this time doth await you” (a minimal change). See under Mosiah 26:23 for an example where the preposition was changed (from in to unto) so that the prepositional phrase would no longer be displaced since it would be interpreted differently. For a general discussion and a list of examples of this usage, see under displaced prepositional phrases in volume 3. Here in 3 Nephi 17:2, the easiest solution would probably be to place a comma after the restrictive relative clause but not before it.
Summary: Maintain in 3 Nephi 17:2 the placement of the preposition phrase “at this time” at the end of the sentence, but add some minimal punctuation (such as a comma after the relative clause) to show that the prepositional phrase belongs to the earlier main clause rather than to the immediately preceding relative clause.