If one were to compare the Book of Jonah with the narrative of what happened to the Messiah Yahshua, then one can understand exactly what the Messiah meant by the 3 Days & 3 Nights scenario:
Now the Lord had prepared a great fish to swallow up Jonah. And Jonah was in the belly of the fish three days and three nights.
For as Jonah was three days and three nights in the whale’s belly; so shall the Son of man be three days and three nights in the heart of the earth.
Then Jonah PRAYED unto the Lord his God out of the fish’s belly, And said, I cried by reason of mine affliction unto the Lord, and he heard me; out of the belly of hell cried I, and thou heard my voice.
Matthew 26:39(Night #1)
And he went a little farther, and fell on his face, and PRAYED, saying, O my Father, if it be possible, let this cup pass from me: nevertheless not as I will, but as thou wilt.
Jonah 2:3 For thou had cast me into the deep, in the midst of the seas; and the floods compassed me about: all thy billows and thy waves passed over me.
Matthew 26:47(Night #1)
And while he yet spoke, lo, Judas, one of the twelve, came, and with him a great multitude with swords and staves, from the chief priests and elders of the people.
Jonah 2:4 Then I said, I am cast out of thy sight; yet I will look again toward thy holy temple.
Matthew 26:57(Night #1)
And they that had laid hold on Jesus led him away to Caiaphas the high priest, where the scribes and the elders were assembled.
Jonah 2:5 The waters compassed me about, even to the soul
Matthew 26:67(Day #1)
Then did they spit in his face, and buffeted him; and others smote him with the palms of their hands,68 Saying, Prophesy unto us, thou Christ, Who is he that smote thee?
Jonah 2:5 : the depth closed me round about, THE WEEDS WERE WRAPPED ABOUT MY HEAD.
Matthew 27:29(Day #1)
And when they had platted a CROWN OF THORNS, THEY PUT IT UPON HIS HEAD, and a reed in his right hand: and they bowed the knee before him, and mocked him, saying, Hail, King of the Jews!
Jonah 2:6 I went down to the bottoms of the mountains;
Matthew 27:33(Day #1)
And when they were come unto a place called Golgotha, that is to say, a place of a skull…
Jonah 2:6 the earth with her bars(Hebrew word also means FUGITIVES) was about me for ever:
Matthew 27:38(Day #1)
Then were there two THIEVES crucified with him, one on the right hand, and another on the left.
Jonah 2:7 When MY SOUL FAINTED within me I remembered the Lord: and my prayer came in unto thee, into your holy TEMPLE.
Matthew 27:50(Day #1)
Jesus, when he had cried again with a loud voice, YIELDED UP THE SPIRIT.
51 And, behold, the veil of the TEMPLE was rent in twain from the top to the bottom; and the earth did quake, and the rocks rent;
Jonah 2:8 They that observe lying vanities FORSAKE THEIR OWN MERCY.
Matthew 27:41(Day #1)
Likewise also the chief priests MOCKING HIM, with the scribes and elders, said, 42 He saved others; himself he cannot save. If he be the King of Israel, let him now come down from the cross, and we will believe him. 43 He trusted in God; let him deliver him now, if he will have him: for he said, I am the Son of God.
Jonah 2:9 But I will sacrifice unto thee with the voice of thanksgiving; I WILL PAY THAT THAT I HAVE VOWED. Salvation is of the Lord.
Matthew 27:46(Day #1)
And about the ninth hour Jesus cried with a loud voice, saying, Eli, Eli, lama sabachthani? that is to say, My God, my God, why hast thou forsaken me?
Psalm 22:25 My praise shall be of thee in the great congregation: I WILL PAY MY VOWS before them that fear him.
Jonah 2:6 yet hast thou brought up my LIFE FROM CORRUPTION, O Lord my God.
Jonah 2:10 And the Lord spoke unto the fish, and it vomited out Jonah upon the dry land.
Friday Night was Night #2, when he was in the tomb.
Saturday Day was Day #2. He was still in the tomb resting on the Sabbath.
Saturday Night was Night #3. He was still in the tomb.
Matthew 28:2(Day #3)
And, behold, there was a great earthquake: for the angel of the Lord descended from heaven, and came and rolled back the stone from the door, and sat upon it…6 He is not here: for he is risen, as he said. Come, see the place where the Lord lay.7 And go quickly, and tell his disciples that he is RISEN FROM THE DEAD; and, behold, he goes before you into Galilee; there shall ye see him: lo, I have told you.
We are told over and over that the third day would be the resurrection:
22 And while they abode in Galilee, Jesus said unto them, The Son of man shall be betrayed into the hands of men:
23 And they shall kill him, and the third day he shall be raised again. And they were exceeding sorry.
18 Behold, we go up to Jerusalem; and the Son of man shall be betrayed unto the chief priests and unto the scribes, and they shall condemn him to death,
19 And shall deliver him to the Gentiles to mock, and to scourge, and to crucify him: and the third day he shall rise again.
33 Saying, Behold, we go up to Jerusalem; and the Son of man shall be delivered unto the chief priests, and unto the scribes; and they shall condemn him to death, and shall deliver him to the Gentiles:
34 And they shall mock him, and shall scourge him, and shall spit upon him, and shall kill him: and the third day he shall rise again.
33 And they shall scourge him, and put him to death: and the third day he shall rise again.
The Messiah told us plainly that he would be resurrected on the third day. If he were to spend 72 hours in the tomb, then he would have been resurrected on the fourth day.
Therefore, since we know the third day was the resurrection day, then all we have to do is count backwards the remaining two days and three nights in order to understand when the count began.