Adam Clarke on Genesis 29:3

All the flocks - Instead of העדרים hadarim, flocks, the Samaritan reads haroim, shepherds; for which reading Houbigant strongly contends, as well in this verse as in Gen 29:8. It certainly cannot be said that all the flocks rolled the stone from the well's mouth, and watered the sheep: and yet so it appears to read if we prefer the common Hebrew text to the Samaritan. It is probable that the same reading was originally that of the second verse also.

And put the stone again upon the well's mouth - It is very likely that the stone was a large one, which was necessary to prevent ill-minded individuals from either disturbing the water, or filling up the well; hence a great stone was provided, which required the joint exertions of several shepherds to remove it; and hence those who arrived first waited till all the others were come up, that they might water their respective flocks in concert.