The original manuscript probably had shewed here, even though the compositor for the 1830 edition ended up setting showed. In the printer’s manuscript, Oliver Cowdery initially wrote showed, but then virtually immediately he corrected it to the archaic shewed. There seems little reason for Oliver to have made this correction unless 𝓞 itself read shewed, especially since we have no specific examples where Oliver permanently switched shew and show in his copywork. For instance, there are 42 cases where 𝓞 is extant for either shew or show, and in all 42 cases Oliver copied the text into 𝓟 without mixing up the two word forms (40 are shew and 2 are show). Only once in the manuscripts did Oliver momentarily mix up the two forms, namely, here in Helaman 15:3 when he initially wrote showed in 𝓟, then virtually immediately corrected it to shewed.
In striking contrast to Oliver Cowdery’s accuracy in copying shew and show, there are nine clear cases where the 1830 compositor mixed up shew and show when he set the type. Five times he replaced an earlier shew with show:
And four times he changed an earlier show to shew:
In two more cases, here in Helaman 15:3 and in 3 Nephi 19:2, the verb form is shew in 𝓟 but show in the 1830 edition:
For these two cases, it is highly probable that the 1830 compositor is the one responsible for the variation, not Oliver Cowdery. The critical text will assume so; thus Helaman 15:3 will read shewed and 3 Nephi 19:2 shew.
The form shewed here in Helaman 15:3 is the past participial form. An alternative form is shewn (or if the past participle is showed, the alternative form is shown). In the original text, out of 38 instances of the past participle, 32 took the -n ending. Of those, only one was shown in the earliest text (in 1 Nephi 14:26); the rest were shewn. Of the remaining 6 instances ending in -ed, only one has ever been changed to the -n ending, namely, here in Helaman 15:3, but only in the 1902 and the 1906 LDS editions (in those two editions showed was replaced by shown, probably independently). There is one case where Oliver Cowdery momentarily wrote shewed in 𝓞 but then immediately corrected it to shewn:
For each case of the past participle, the critical text will follow the earliest textual sources, thus shewed here in Helaman 15:3, not shewn (or the later shown). For a complete listing of all the cases of shew and show, see under shew in volume 3.
Summary: The original manuscript in Helaman 15:3 very likely read shewed (the corrected reading in 𝓟), not showed (the reading in the 1830 edition); there is no evidence that Oliver Cowdery ever permanently mixed up these two verb forms when he copied from 𝓞 into 𝓟, but the 1830 compositor frequently did; thus we accept the reading in 𝓟 as the correct reading in Helaman 15:3 (and similarly in 3 Nephi 19:2).