Your saying,I read a commentary somewhere that mentioned "three days and three nights" was idiomatic and basically meant simply "three days", which colloquially meant "any portion of three consecutive days".
Once upon a time I struggled with this but It's now plain Jesus died Friday afternoon and rose either late Saturday after the sun had set or early Sunday before the sun rose.
Jesus died Friday afternoon , to sat afternoon would be one day one night, and rose on that same day on Saturday or early Sunday morning?
In my estimation that's about one and a half days , and one night. i don't consider that idiomatic.
The information I'm giving you is from over 50 years of biblical study by one who owned the second largest collection of bible manuscripts in private hands, right behind the Vatican. He was fluent and taught from all the ancient texts that he had at his finger tips.
I don't mind disagreement , but try to present a better understanding based upon biblical facts.
jay