Sforno on Numbers 22:22/ספורנו על במדבר כ״ב:כ״ב

1כי הולך הוא, that his purpose in accompanying them was not to lead others as in Kings II 4,30 where Gechazi followed the dead boy’s mother to be shown the way, etc. Bileam, on the other hand, went as if he had his own agenda, attempting to foil the will of the Lord. The delegates had not come to ask for his advice at all.

2לשטן לו, to obstruct him. The subject matter known as שטנה, always involves an activity contrary to someone else’s activity. We find an example of this expression in this sense in Genesis 26,21 where the Philistines foiled the attempt of Yitzchak’s servants to use the water from the well they had dug. In this instance the angel had come to oppose Bileam seeing his journey contravened G’d’s will. He was afraid that Bileam would do his usual and not be content to await a propitious moment, so that he would become guilty of death. G’d wanted to spare him such a sin. [if I understand the author correctly, he feels that Bileam’s function as a “prophet”, i.e. pronouncing curses, when exploited at times when doom threatened his potential victims anyway, would not be construed as sinful by G’d. Ed.]

3והוא רוכב על אתונו ושני נעריו עמו. This is why he did not see the angel; as our sages explain when three people travel together they do not see potential dangers but they will not be harmed by what they do not see either.