Is it maybe『ふつ、みっかのうちに』or『に、さんひのうちに』? (second one would be really weird though)
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It seems like it would be read 「にさんにち」
When I type that reading in, it automatically changes it to 「二、三日」
Additionally, there’s an entry on Jisho without the comma that shows that as the reading and I also found this : kanji - How to read 二、三日 - Japanese Language Stack Exchange
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That makes sense; thank you!
@Gacee already explained the correct reading and in general things like this will become more obvious the more you read, but I feel there is a bit of a rule here.
When you have a series of kanji with different phonetics/readings, think of the meaning the kanji is trying to convey. Here it’s “2, 3 days”. In that case you wouldn’t read it “second, third day”, because that would mean something else and grammatically it wouldn’t work.
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I’m about to finish my 12th (or 13th?) novel, and it’s pretty common. But I think I realized I always ran with an assumption I didn’t further question. That’s one of the things that are so difficult about reading in Japanese (without furigana). Maybe you’ve internalized the wrong readings but you won’t realize for a somewhat long time. 
That being said, 二日 and 三日 can just mean “two days” and “three days” respectively, according to https://www.goo.ne.jp/. So the the implication doesn’t necessarily need to contain ordinality as far as I’ve understood. I may be wrong though.
For reference:
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Yes, that’s what I meant 
. The ordinality would be clear from context and if there is non-zero ambiguity, the author sometimes adds furigana to make it extra clear. At least that’s what I’ve seen in other cases in light novels.