A: Bless you for having three young boys! It is wonderful that they are not strangling one another all day long. Sharing is hard for kids, and learning to be unselfish is a lesson so difficult that even many adults haven't mastered it.
Unfortunately, you can't teach kids to be unselfish; little children are selfish, and it is very likely that the brothers are trying to hog your 8-year-old's present so the birthday boy hardly gets an opportunity to play with it himself. While he does need to learn good manners that involves offering others a chance to play, the meaning of a "present" is that it is presented to him, not to all the siblings as a group.
If the family owned a pony, let's say, you could insist that all the boys take turns. But I don't blame your son for feeling confused about how often he is supposed to hand over his present to his brothers, who are probably clamoring for it non-stop. Either tell your sons that the present belongs to the 8-year-old (period) or find a way to let each boy have his own Nerf gun so they can play together. You can avoid endless cajoling, nagging, and negotiating over who is allowed to use one Nerf gun and for how long.