As with 99% of these types of seeming “contradictions”, when THE DAY finaly does arrive, we’ll see that the 3-day warning was probably something that was “heard” every day, but ignored.
Either that, or it will be some math mis-calculation like the way the “400 years in Mitzrayim” was really only 210 in the land itself, 69 of them slavery. So when Moshe came around and said, its geulah time, there were no doubt skeptics that said, “can’t be… its not 400 years yet”.
Either way, let the geulah shelaymah begin, and we’ll worry about the details later!