Adam Clarke on Exodus 3:18

They shall hearken to thy voice - This assurance was necessary to encourage him in an enterprise so dangerous and important.

Three days' journey into the wilderness - Evidently intending Mount Sinai, which is reputed to be about three days' journey, the shortest way, from the land of Goshen. In ancient times, distances were computed by the time required to pass over them. Thus, instead of miles, furlongs, etc., it was said, the distance from one place to another was so many days', so many hours' journey; and it continues the same in all countries where there are no regular roads or highways.