John Gill Commentary Judges 9:43

"And he took the people, and divided them into three companies, and laid wait in the field; and he looked, and, behold, the people came forth out of the city; And he rose up against them, and smote them." — Judges 9:43 (ASV)

And he took the people
That is, the forces he had with him at Arumah:

and divided them into three companies :
each having a separate leader, and the command of one of them he had himself:

and laid wait in the field ;
in the field of Shechem, one company in one part, and one in another part of the field:

and looked, and, behold, the people were come forth out of the city ;
he watched them when they did:

and he rose up against them, and smote them ;
the companies rose up out of their ambush, in different parts, and killed them.