Friend, Jesus did not say it would be 3 days and 3 nights from the time he died to his resurrection--
but from the time He would be in the heart of the earth, the tomb---and He fulfilled it to the minute---because HIS Word is Truth !
The phrase "heart of the earth" I would think it would have to be defined. I think if He wan't to convey in the grave or tomb he would of used those worlds. And if we compare it to the parable in Jonas we should take into account that Jonas was not dead but suffered three days.
When did the work of spiritual suffering begin? Or when did he first start pouring out His Spirit on flesh according to the promise?.