Horrible experience with rabbits! Help!?

Well, we had two adopted rabbits that we got fairly young. We got them both together, and they seemed to get along fairly well for the time we had them. We fed them vegetables, pellets, let them run and hop around, gave them a large space for their cage so they had room to stretch and move still, petted them, cleaned their area often. They were also both neutered/spayed (one male, one female). It was fairly easy to deal with the both of them. Then, we got another bunny who wasn't adopted, because we thought that they would get along very well with it. This bunny is a tiny baby, and adorable as can be. An all black little lop ear. x3 He seemed so curious about the other two bunnies, so we tried bonding them and seeing how they reacted to one another. He started to binky, which we thought it was a good sign, but then the other male tried to attack him, and the girl bunny seemed disinterested in him, so we decided to put the baby bunny in a separate area next to theirs. The older male rabbit tried to claw at and lunge at the baby rabbit still and tried to get out of his cage to get to him, and about a week ago, he suddenly died while still facing the area where the baby bunny's cage was. We were so confused by this because we still gave them all equal attention, let them run around, and we make sure they have plenty of food and water. Now that the older male is gone, we tried to see how the female and the new rabbit got along, and at first it seemed to be going well, but now there seems to be a problem. She's lunged at him a couple of times and acts aggressively toward him. We don't know what to do.... Help?