Usually I'd say that keeping a stallion, mare and gelding all together is a recipe for disaster. It still could be. Or it may not. I currently have a large pony sized Arab mare out with our geldings. Originally, the two stallions were out with them as well but they are currently separated. They were separated not due to personality problems but due to fencing issues.
in the past, I've run our stallions with a group of mares. The paddocks or pastures DO need to be big enough to allow for natural movement and for the horses to each get away from one another if they want to. Some stallions will kill a foal, others are great "babysitters".
Our first stallion was AWESOME with the foals - even allowing a foal to "suckle" on him if the foal thought that was where the milk was - until the foal was approximately a year of age. THEN, that foal - either sex - was driven out of the group - just like in a wild horse/mustang type of group. If we didn't remove that youngster at that point OR if the youngster was left but ignored stallion's "orders" to leave the group, he would become serious enough in driving them away as to cause injury. We had one yearling filly get out of a pen and try to return to her dam. When she ignored his efforts to "nicely" drive her away, he grabbed her by a hind leg, literally yanking her off the ground and slinging her sideways. At that point, we had a tooth laceration and some strained tendons to deal with in her - we were able to get her caught. I also watched him gradually change towards a young gelding with each spring month past one year of age until he was literally attacking him and pinning him along fences. If I'd left that pony in the pasture with him, I have no doubt that eventually I'd have had a dead gelding. When we were ready to wean foals, I could put him in with the newly weaned foals and they'd be comforted by his presence and he would itch and scratch them. I usually removed him and put him back out with the mares after a couple of months.
Our 2nd stallion was unrelated to the first and appeared to have never been out with mares/foals. He would round up and drive the playing foals into a tight group with the mares at first. He didn't like a foal approaching him - he'd pin his ears and leap at the foal with "gnashing" teeth, running them down and running right over the top of them. As he "got used to" the foals, he quit doing that and the following year, he didn't do it so much. Again though, he wasn't accepting of the previous years foals at all once they were around a full year of age. I did end up putting this stallion out with an arab mare and her arab gelding for over a year. If the gelding had been more domineering, i don't think it would have worked. The stallion got a lot of practice at herding - he herded them both...