There is no reason to think the 3 days has anything to do with the gospel. Many elements of life and many cultures throughout history use the "rule of three." By suggesting 3 days, Esther is show the grave importance of this fast. One or two days wouldn't be enough. Three days shows sincerity and devotion.

[quote]15Then Esther sent this reply to Mordecai: 16“Go, gather together all the Jews who are in Susa, and fast for me. Do not eat or drink for three days, night or day. I and my attendants will fast as you do. When this is done, I will go to the king, even though it is against the law. And if I perish, I perish.”[/quote]
Esther 4 v. 15-16 NIV

Do you think the three days and three nights, have to do with the gospel of Christ? Or is this just a coincidence?