Here scribe 2 of 𝓟 omitted the now when he copied the text from 𝓞 into 𝓟. Oliver Cowdery later supplied it when he proofed 𝓟 against 𝓞. The 1830 edition, set from 𝓞, also has the now; thus we can be confident that 𝓞 also had the now. See under Helaman 7:29 for another instance where the scribe (in that case, Oliver Cowdery) omitted the original now after behold, at least initially, when he copied the text from 𝓞 into 𝓟.
For this part of the text where both 𝓟 and the 1830 edition are firsthand copies of 𝓞, I will usually not discuss those cases where the variation consists solely of a correction by Oliver Cowdery to scribe 2 of 𝓟’s reading (or scribe 2’s own correction), providing the corrected reading agrees with the 1830 reading. In volume 3, I list all these cases of manuscript variation between 3 Nephi 19:21 and Mormon 9:37, where scribe 2 of 𝓟 was the main scribe in 𝓟 and the 1830 edition was set from 𝓞; unfortunately, 𝓞 is extant for only three minor fragments in this part of the text.
Summary: Maintain the now in 3 Nephi 20:10 (“behold now I finish the commandment”), the 1830 reading and Oliver Cowdery’s corrected reading in 𝓟.