Is Trinity a divine mystery? If so, one has to believe in something mysterious (unexplainable) to get salvation?
Your question is an interesting one in that it assumes that one is in need of salvation. Not all religions believe that this is necessary. In general, only those who believe in the “Fall of Man” and hereditary sin require salvation. If you do not believe that you are in a “Fallen” state, you do not require salvation. In other words, you have to believe in something mysterious and unexplainable to believe you are in need of salvation. Trinity is an interesting concept. Three is a powerful number in the mysteries, and many trinities have existed. Mother-Father-Son is common (Isis, Osiris, Horus, for example), as is Maiden-Mother-Crone (as differing aspects of the same Goddess). In Hinduism, the masculine God form is seeing as Creator-Preserver-Destroyer (Brahma-Vishnu-Shiva). In Christianity, the concept of Father-Son-Holy Spirit as aspects of the same God is common.