Here the adverb up has been lost at two different times, in 𝓟 by scribe 2 and later in the 1874 RLDS edition. The RLDS text has maintained the shorter reading. Oliver Cowdery, when he proofed 𝓟 against 𝓞, restored the up. The critical text will maintain the up after the verb stand. For further discussion regarding the loss of up before upon, see under Alma 2:15.
Summary: Maintain in 3 Nephi 20:2 the adverb up in the conjoined predicate “and stand up upon their feet”, the reading of the earliest textual sources (the 1830 edition and the corrected reading in 𝓟).