Here the 1830 compositor initially set the name Giddianhi as Gaddianhi, undoubtedly under the influence of the name Gaddianton, which occurs along with the first occurrence of the name Giddianhi in the text:
As discussed under Helaman 2:4, the name Gaddianton ended up being spelled in the printer’s manuscript with one d, as Gadianton, but there is considerable evidence that in the original manuscript the spelling Gaddianton prevailed. Since for this part of the text the 1830 edition was set from 𝓞, it is not surprising that the typesetter sometimes allowed Gaddianton to affect his spelling of the name Giddianhi, namely, as Gaddianhi. Here in 3 Nephi 3:12, the 1830 compositor later corrected Gaddianhi to Giddianhi in an in-press change. He made this same mistake once more in his typesetting:
Although this second misspelling occurred in the same 1830 signature (number 29) as the first one (in 3 Nephi 3:12), it was never corrected in-press. Otherwise, the 1830 compositor set the name Giddianhi correctly (11 times).
Summary: Maintain the spelling Giddianhi throughout the text, including the two cases in 3 Nephi 3:12 and 3 Nephi 4:9 where the 1830 compositor mis-set the name as Gaddianhi under the influence of the name Gaddianton.