I hold the view it is three whole days and three whole nights, ie partial days or nights don't count.
As such I am of the view Good Friday is in fact Good Thursday. The first day of the Passover Week, the 15 of Nissan, was a Friday, a Sabbath. Thus no one could go to Jesus' tomb until Sunday morning.
But there is still a problem. For counting whole days and nights, ie 12 hour periods, if Jesus died before Thursday evening, then three whole nights have passed: Thursday, Friday and Saturday. But only two whole mornings, namely, Friday, and Saturday. Jesus was up and about on Sunday morning, after just two days. In fact he was up "while it was still dark" [Jn 20:1], and that means he could have resurrected as early as the dusk on Saturday, and missing one whole night.
In other words Jesus could have been in the "heart of the earth" for only two whole days and nights, and what then about the remaining one more day and night? To understand that we need to review our understanding of what is a day and what is a night, ie it may not be literal.