Hello Mike 1988, and welcome to CC.
Wow talk about a difficult question.
The Trintiy is rather difficult to understand. I believe the best way to describe the Trinity is Three Divine Being, each being God, but not three different God(s). Some will say this can't be explain. But I think it can be explained when one looks at the United States Government. I notice ( if I recognize the flag by your name right) that you are from Great Britian, so I don't know how much you know of our Government accross the pond from you. But here in the the US we have three branches to our government. Each branch is the Government, but not three different governments, but one united federal government. So likewise do I believe the Trinity/Godhead to be (Acts 17:29; Colossians 2:9). Just as we in the United States have a Legisative branch called the Congress which is the US Government and properly called the Government but not a separate government from the other two branches; so God the Father (Yahweh) is God, and properly called God, but not a seperate God, from other Two members of the Godhead. Just as Christ Jesus the Son of God is God, and properly called God, but not a separate God, from the other Two Members of the Trintiy, so the Judicial Branch (the Courts/Supreme Court) of our US Government, is the government, but not a seperate government. And just as the executive branch (the President) of the US Government is the government, but not a separate government, so the Holy Spirit is God, yet not a separate God from the Father and the Son. For we read many times in the Bible of God/the Godhead referring to themselves in the plural form (Genesis 1:26; 11:5-7; Isaiah 6:8; 44:6).
Isaiah 33:22
For the LORD is our Judge, the LORD is our Lawgiver, the LORD is our King:; He will save us.
I John 5:7
For there are three that bear record in heaven , the Father, the Word, and the Holy Ghost: and these three are one.
And yes the above is authenic Scripture, for enjoys God eternal protection thru His own Word.
So why did Jesus died for sins. I think that NMsmile gave a good example but I will add, that the Godhead is just, and being just they cannot accept sin, without some type of payment. God Wrath will one day come down on all who sin, unless they have accepted the Blood of Christ.
Romans 5:8-9
8) But God commendeth His love for us, in that while we were yet sinners, Christ died for us.
9) Much more then, being now justified by His Blood, we shall be saved from wrath through Him.
So what happen to though who came before Jesus. We read in Psalms 16:10-11; Acts 2:25-36, that Jesus went to hell/prison we read in the following Scripture what Jesus did while He was in prison.
I Peter 3:19
By which also He went and preached unto the spirits in prison.
I think the Word of God tells us here that Jesus preached to though who had died before He came. So that God could not be called unjust in any manner at all (I Peter 4:6).
I apologize about the long post, but hopefully it will help your understanding.