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two days before yesterday; three days back (ago)
一昨々日から風邪を引いて寝込んでいる。
I've been bedridden with a cold since three days ago.
一昨々日届いた荷物をまだ開けていない。
I still haven't opened the package that arrived three days ago.
一昨々日の天気は今日より暑かった。
The weather three days ago was hotter than today.