Adam Clarke on Joshua 1:11

Prepare you victuals - צדה tsedah, such prey or provisions as they had taken from the conquered countries, such as corn, oxen, sheep, etc.; for the word signifies prey, or what is taken by hunting, etc. This was necessary, as they were about to undergo considerable fatigue in marching, and in making preparations for the passage of the Jordan; for although the manna had not ceased to fall, yet such other provisions as are mentioned above were necessary on this occasion.

For within three days ye shall pass - Calmet contends, with great appearance of truth, that these three days should be reckoned from the first day of their encamping at Jordan, three days after the return of the spies, i.e., on the eighth day of the first month, on the tenth of which they passed over Jordan. The text therefore is supposed to mean, Prepare victuals for three days' march, for "on the third day after your decampment from Shittim ye shall pass over this Jordan."