also: さおととし · さいととし · いっさくさくねん
three years ago; the year before last year's last year
Refers to the year three years prior to the current year. Can be used as a noun or adverb. The reading さきおととし is most common in speech; いっさくさくねん is formal and literary.
The summer three years ago was very hot.
He graduated from university three years ago.
Contracted colloquial reading; less common.
Dialectal or archaic variant reading; rarely used.
Formal Sino-Japanese reading; mostly encountered in written or formal contexts.