"So OT saints were sleeping until Jesus rescued 'captivity captives' (OT saints) during His three days while his body was in the tomb."
Christ was in the belly of the earth for the three days you refer to. He actually ascended to Heaven after he resurrected from the dead. Remember after his resurrection He met Mary in the Garden, and told her not to touch Him because he had not yet ascended to His Father? Then, later that day He met with His disciples, after His ascension? At this time, he allowed His Disciples to see his wounds, etc. to prove to them that he was not a spirit, but flesh and bone. Then, 8 days later, Thomas placed his hands on Christ wounds. Christ could not be touched until after the ascension, that took place immediately after her saw Mary in the Garden:
" 15. Jesus saith unto her, Woman, why weepest thou? whom seekest thou? She, supposing him to be the gardener, saith unto him, Sir, if thou have borne him hence, tell me where thou hast laid him, and I will take him away.
16. Jesus saith unto her, Mary. She turned herself, and saith unto him, Rabboni; which is to say, Master.
17. Jesus saith unto her, Touch me not; for I am not yet ascended to my Father: but go to my brethren, and say unto them, I ascend unto my Father, and your Father; and to my God, and your God.
18. Mary Magdalene came and told the disciples that she had seen the Lord, and that he had spoken these things unto her.
19. Then the same day at evening, being the first day of the week, when the doors were shut where the disciples were assembled for fear of the Jews, came Jesus and stood in the midst, and saith unto them, Peace be unto you.
20. And when he had so said, he shewed unto them his hands and his side. Then were the disciples glad, when they saw the Lord.
21. Then said Jesus to them again, Peace be unto you: as my Father hath sent me, even so send I you.
22. And when he had said this, he breathed on them, and saith unto them, Receive ye the Holy Ghost: 23. Whose soever sins ye remit, they are remitted unto them; and whose soever sins ye retain, they are retained.
24. But Thomas, one of the twelve, called Didymus, was not with them when Jesus came.
25. The other disciples therefore said unto him, We have seen the Lord. But he said unto them, Except I shall see in his hands the print of the nails, and put my finger into the print of the nails, and thrust my hand into his side, I will not believe.
26. And after eight days again his disciples were within, and Thomas with them: then came Jesus, the doors being shut, and stood in the midst, and said, Peace be unto you.
27. Then saith he to Thomas, Reach hither thy finger, and behold my hands; and reach hither thy hand, and thrust it into my side: and be not faithless, but believing.
28. And Thomas answered and said unto him, My Lord and my God."
There are some different views of this, but one of the major ones that I agree with, is that OT saints were to be brought to heaven by Christ, after He shed His blood on the cross, whereas we immediately go to heaven upon death. Their sins were COVERED by the blood of animals, but Jesus' blood CLEANSES from all sin. So OT saints were sleeping until Jesus rescued 'captivity captives' (OT saints) during His three days while his body was in the tomb.
We have done quite a few threads on this topic, and agreed and disagreed over the points, so this is just one view of many (as far as I know).
Both OT and NT saints are saved by grace through faith. OT saints to the future Christ, NT saints (us) by the person and completed work of Christ.