So the sun sets and then sets again on Thursday. One day. Then it sets on Friday. Two days. Then it sets on Saturday. Three days "in the heart of the earth." Then, after the sun sets on Saturday - in other words, on the fourth day - He rises.
We are left with the same problem: everywhere else in Scripture He rises on the third day, not the fourth.
Starting the timeline on Wednesday does not resolve that difficulty. If He is "in the heart of the earth" for three entire chronological days, He does not rise on the third day - He is "in the heart of the earth" that entire day.
Where does the Scripture say he rose on the 'fourth day'? It says the women went to check before the dawn on the first day (which would be the fourth day, from Wednesday) and He was not in the tomb.
Christ left the tomb without the seal being broken as described when the tomb was opened and He was not inside. How do we know His transformed body could do so... well He showed doubting Thomas that His transformed body could go through a literal wall.
Your own math is wrong on this. 6PM Thursday is day one
6PM Friday is day two
6PM Saturday is day three. Jesus' tomb was found empty already at 6 AM on Sunday, 12 hours after 6PM Saturday.