Do you know how to diagram a sentence ? If not get someone who knows how to diagram a sentence for you then you will understand that the “it” in Romans 4:3 is Abraham’s believing God. In Romans 4:9 where it says faith was reckoned to Abraham it is saying God credited to Abraham by grace through faith righteousnesses because he believed in the messiah and it is by grace through faith in the finished work of the Messiah that you and I are credited( reckoned) with righteousness.
No, the "it" was not Abraham believing God.
Cometh this blessedness then upon the circumcision only, or upon the uncircumcision also? for we say that
faith was reckoned to Abraham for righteousness.
Romans 4:9 KJV
Faith here is the substance of the Gospel, which is the person and work of Jesus Christ, described in the previous chapter of Romans 3
Where is boasting then? It is excluded. By what
law? of works? Nay: but by the
law of faith. Therefore we conclude that a man is
justified by faith without the deeds of the law.
Romans 3:27-28 KJV
From here Paul uses faith as a metynomy for Christ, or the "law of faith".
You can deny it all you want but it was not Abraham's faith which justified Him. It was the shed blood of Christ that justified Abraham.
Much more then, being now
justified by his blood, we shall be saved from wrath through him.
Romans 5:9 KJV
Abraham was not justified until the death of the testator.
And for this cause he is the mediator of the new testament, that
by means of death, for the redemption of the transgressions that were under the first testament, they which are called might receive the promise of eternal inheritance. For where a testament is, there must also of
necessity be the death of the testator. For a testament is of force after men are dead: otherwise it is of no strength at all while the testator liveth.
Hebrews 9:15-17 KJV
Abraham received the promise but they were fulfilled in Christ.
These all died in faith,
not having received the promises, but having seen them afar off, and were persuaded of them, and embraced them, and confessed that they were strangers and pilgrims on the earth.
Hebrews 11:13 KJV
This is why Jesus says:
Your father Abraham rejoiced to see my day: and he saw it, and was glad.
John 8:56 KJV