Here 𝓞 is extant and reads “for the space of three days and three nights”. When Oliver Cowdery copied the text into 𝓟, he accidentally omitted the repeated number three, probably because that is what we expect in English (“three days and nights”). Later Oliver supralinearly inserted in 𝓟 the repeated three with somewhat heavier ink flow, perhaps when he proofed 𝓟 against 𝓞.
The Book of Mormon text always repeats the plural number in conjuncts involving days
and nights:
In fact, there is a similar kind of repetition involving the number one, although in that case the repeated number is not one but the indefinite article a (which, it turns out, is historically derived from the number one):
For a general discussion of this kind of repetition, see under conjunctive repetition in volume 3.
Summary: Maintain the repeated three in Alma 36:10 (“and it was for the space of three days and three nights”), the reading in 𝓞 and the corrected reading in 𝓟.