Unfortunately, even neurotypical siblings do a lot of fighting. Adding ADHD to the mix means adding impulsivity (no holding back) and a need for stimulation that can show up as picking fights and such.
Family therapy or some professional social skills help might be necessary, to teach your children to specifically get along with each other. If there are activities they all enjoy, do those as much as possible and then have them do some individual things alone in between. No siblings can stand to be together every moment.
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism