一昨昨日 (issakusakujitsu) means “three days ago”

More examples

一昨昨日の朝、駅で傘を買いました。is saku saku jitsu no asa eki de kasa o ka i ma shi ta

Three days ago, I bought an umbrella at the station in the morning.

Marks a point in time exactly three days before the present day.

三日前mikkamae: A common everyday expression for a point three days before the present day.

今日から三日前の日。kyō ka ra mik ka mae no hi

一昨昨日 (issakusakujitsu) combines 一昨 (issaku, "one before the last") with 昨日 (sakujitsu, "yesterday") to extend the reference back one more day.

一昨昨日 (issakusakujitsu) is a formal-sounding time expression, so everyday Japanese often uses 三日前 (mikka mae, "three days ago") instead.