There is a bug with the companions, specifically Robot. I have four companions at once Dog, Charlotte, Zavvi, and Robot. I think it happened because of how Robot joins you. Robot is immediately put in the party after fixing them (at least that's what happened to me), and adds Robot as a companion without removing any other companion from your group. This has also raised the amount of companions I can have permanently, as I can remove a companion from my group and it would say 2 out of 3 companions. This gives me the option to switch a companion out with another and still keep four in total although this is just speculation because I haven't messed around with this.