Salvation refers to how Jesus offered mankind eternal life through is crucifixion. It is believed that those who have faith in God will have eternal life.
Free will is something that we all have. God gave us the gift of free will. He gave it to every human being. It is what enables us to choose to do or believe what we want. It is up to us as humans to figure out what is right and wrong.
Belief in holy trinity is the faith revealed in True God or a declaration of christian faith in- the Holy Trinity of the Father, the Son, and the Holy Spirit.
God is triune in person, the ones listed above.
For further information of the three persons of holy trinity click here and the information listed above.
For more helpful information on the belief in holy trinity click here. -look for Holy Trinity in bold letters.