John Gill Commentary Joshua 3:12

"Now therefore take you twelve men out of the tribes of Israel, for every tribe a man." — Joshua 3:12 (ASV)

Now therefore take you twelve men
For what end and purpose is not mentioned.

Abarbinel is of the opinion that they were chosen and appointed so that each man might select and take a place for his tribe to encamp in when they came to the other side of Jordan. Whether this was Joshua's view is not certain. However, the use he made of these men, thus provided by divine direction, is related (Joshua 4:2–4).

and these were to be taken out of the tribes of Israel, out of every
tribe a man ;
Levi was not reckoned, having no part in the land; and so we find that tribe left out in other accounts, when there was a choice of twelve men out of each of the tribes, (Numbers 13:4) (34:19) .