Christ died around three in the afternoon on Friday and was entombed shortly thereafter. The Resurrection occurred by dawn on Sunday. Thus he was in the tomb less than nine hours Friday (by the modern reckoning), twenty-four hours Saturday, and less than six hours Sunday–at any rate, far less than the seventy-two hours that comprise three full days and nights.
Is there a contradiction here? No, because the ancient Jews counted as a whole day any part of a day, so “three days and three nights” (which means the same as “three days” in modern usage) could be as little as twenty-four hours plus a few seconds on either side–if there had been, back then, clocks that could register seconds.
In our way of reckoning things, from lunch time today to lunch time tomorrow is one day. Ancient Jews would have counted it as two days because it includes parts of two distinct days.