Respectfully, the sign of Jonah has more to do with the promised resurrection of ALL of mankind than the literal death of Jesus and his spending three days in his grave. Jonah - a man who lived BEFORE Christ - is mentioned here too.
Jesus was speaking of something much greater here at Matthew 12:40: The promised "Great Resurrection" of ALL of mankind. (John 5:28-29)
At Matthew 24:30 - and in answer to the question his disciples asked at Matthew 24:3 - Jesus mentions a sign that will be seen in heaven. That is, a sign seen only by those who reside in heaven. That sign - or single event - is the coronation and enthronement of Christ as King over the earth for 1000 years.
One of his first acts as King over the earth is to resurrect ALL of dead mankind, to include persons born since Adam to include Adam. This is a promise.
No sign will EVER be given to mankind, which has ALWAYS been adulterous in worshiping other gods except the one associated with the one seen in heaven.
Jonah was a man. He came out of the belly of the great fish after three nights. This sign referred to here is ALL of mankind being called out of their graves in the distant future. It shows that a person (dead) can spend time in the grave and come out live!
That Christ spent three days and his nights have no meaning to most people as that happened more than 200 years ago. But it DOES have meaning when it is applied to ALL of mankind for who Jesus died for.
In other words, if Christ died and rose again, so shall all of mankind who have died. Yes, those who died BEFORE Christ came on the earthly scene. The sign of Jonah is the promised resurrection of ALL of mankind.
(See The Apostle Pauls talk to the congregation in Thessalonica at 1 Thess 4:13-18)