Triune refers to something that is made up of three parts or aspects [parts or features of something].
For example, the Christian concept of the Trinity is triune, as it refers to God being comprised of three persons [ways of looking at God, or identities of God]: the Father, the Son, and the Holy Spirit.
Very Simple Definition:
Triune means having three parts or aspects.
For example, a tripod is triune because it has three legs.