It doesn't say she will go AFTER the third day. It says "and thusly" (the Hebrew, u'v'chen means a lot more than "and then" -- Klein's dictionary reads "and so, if so, thus, therefore, well.")
The Talmud records that she survived naturally but other texts posit that one cannot actually fast for three days without some sort of miracle (and one opinion says that there was some eating happening at night (literally, the text says "fast on my behalf night and day" which certain commentators see as establishing a day fast and a night fast which would interrupted by a small amount of food which kept people from passing out.