Why was Lavan informed that Ya'akov had fled only three days later?
Rashi: Because Lavan, to ensure that Ya'akov would not steal his sheep, had placed his flocks with his sons at a distance of three days from those of Ya'akov, and he had just gone there to shear the sheep.
Targum Yonasan: The moment Ya'akov left, shepherds noticed that the well ceased flowing. They waited three days, 1 at which point they concluded that Ya'akov must have left, and informed Lavan 2 to that effect. 3
Moshav Zekenim: The informer did not realize that Yakov was fleeing until the third day after Yakov left and did not return. 4 Lavan was told on the sixth day from when Yakov left.
Hadar Zekenim: The informer immediately went to tell Lavan when Yakov left. It took three days for him to get to where Lavan was and for Lavan to return to Charan.
See explanation 'Na'ar Yonasan'.
Even if Lavan was three days away, i.e. when walking normally, perhaps the informer went on a fast camel and made the entire journey during the third day. (PF)
See also Ba'al ha'Turim.
This is unlike the simple reading of our verse. Yakov took everything, including his vast flocks. Why did the informer realize only after three days that he fled? Perhaps the informer was unaware of Lavan's animosity, and could not fathom that Yakov would flee. Also, if it were a "false alarm", he would waste three days journey, plus three days returning, for himself and Lavan, so he waited until he was sure. (PF)