noun-modifying (irregular)
triennial; three-year (plan, period, etc.)
Used attributively to describe a plan, project, or period spanning three years. Common in government, business, and institutional contexts.
市は三カ年計画を発表した。
The city announced a three-year plan.
この三カ年の間に大きな進歩があった。
There has been great progress during these three years.
Using Arabic numeral 3 instead of kanji 三; common in charts and tables.
Arabic numeral with ヶ variant.
Arabic numeral with hiragana か.
Arabic numeral with kanji 箇.
Arabic numeral with ヵ variant.
Arabic numeral with full-size ケ.
三年 is the general term for 'three years' as a duration, while 三カ年 is more formal and often used in official names of plans or periods.
Compound of 三 (three) + カ (counter for years, derived from 箇) + 年 (year). The counter カ/ヶ/か is a simplification of the kanji 箇.