Immortal soul concept: why does Bible say Jesus was resurrected after 3 days, as he should never have been really dead?

You appear to not be separating Physical death from Spiritual death. Jesus did in fact suffer physical death and he did in fact take up that physical life again.

John 10:17 and 18 KJV Therefore doth my Father love me, because I lay down my life, that I might take it again. No man taketh it from me, but I lay it down of myself. I have power to lay it down, and I have power to take it again. This commandment have I received of my Father.

The life Jesus is referring to here is his physical life.

Matthew 25:46 KJV And these shall go away into everlasting punishment: but the righteous into life eternal.

The life Jesus is referring to here is spiritual life.

So what is the difference between the two? That is where the living soul comes into play spoken of in:

Genesis 2:7 KJV And the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul.

There are two deaths enumerated in the Bible.

Revelation 20:5 and 6 KJV But the rest of the dead lived not again until the thousand years were finished. This is the first resurrection. Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years.

In these Scriptures we are told of both Physical and Spiritual death. Verse 5 deals with physical (first) death, while verse 6 deals with Spiritual (second) death. We are all very aware of the fist or physical death, but few are well versed on the Spiritual or second death. So let us define the second death as enumerated in the Bible.

Revelation 20:11 through 15 KJV And I saw a great white throne, and him that sat on it, from whose face the earth and the heaven fled away; and there was found no place for them. And I saw the dead, small and great, stand before God; and the books were opened: and another book was opened, which is the book of life: and the dead were judged out of those things which were written in the books, according to their works. And the sea gave up the dead which were in it; and death and hell delivered up the dead which were in them: and they were judged every man according to their works. And death and hell were cast into the lake of fire. This is the second death. And whosoever was not found written in the book of life was cast into the lake of fire.

So actually death is a misnomer in that death is also in the lake of fire so there can be no more Physical death. This leads us to an explanation of what living soul means and exactly what eternal life indicates.

Life is a part of God as we see in:

Genesis 2:7 KJV And the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul.

there are two very cogent statement we need to understand here;

and breathed into his nostrils the breath of life,

The breath of life is from and is a part of God himself. and the second is;

Man was created, but did not have life until God breathed it into his nostrils. So if the breath of life is a part of God it is also eternal in that it had no beginning and will have no end.

And so the only thing left is spiritual life and the second death actually means the eternal destroying of eternal life (the soul) in fire.

Although this is somewhat off subject we know from our study of the Bible that fire is a purifying agent. Therefore, The soul will be purified either by the sacrifice of Jesus and our acceptance of that purification or purification through the lake of fire. It is also interesting to note that the Devil and his angels are also a part of that eternal purification, even though they, as will those who do not accept salvation, never attain full purification, and will spend eternity in the lake of fire.

Revelation 20:10 And the devil that deceived them was cast into the lake of fire and brimstone, where the beast and the false prophet are, and shall be tormented day and night for ever and ever.

So these are the differences between the two deaths that the Bible enumerates and is the basis of Christianity and where the word saved comes into play in that Jesus saved the elect from eternity in the lake of fire.

Hope this helps.