We didn't have quite that range but it really wasn't that hard. When my eldest went at 6, the other two were 3 and under 1. When my eldest went at 8, the other two were 5 and 2.
I *think* the official age for riding alone is 8. "Alone" just means without an adult next to them. Since not all rides have enough room across, that will help you out quite a bit, meaning it won't get in your way on any ride except Speedway. The issue with Speedway is that one has to be a certain height to go alone and not all 8 year olds are going to reach that height. So, if that's a priority, figure out how to do that one if they aren't tall enough to go alone.
Child swap is easy and will make the child not have to wait in line. If the child is not tall enough, the child can't wait in line. We only ran into issues when we had a child that was tall enough but didn't want to ride - and they wouldn't let us do rider swap. Luckily, they did on our last trip, but there was one trip where they wouldn't let us do rider swap with a 40" 3 year old who was too scared to do the rides. Hopefully, that issue is a thing of the past.
I have nursed all over Disney. Our first with-kids trip included a 2 month old. We've had other trips with nurslings who nursed frequently. Babies are generally easier to travel with than 3 year olds. My kids travelled in the sling at that age and it worked out very well. We brought a stroller that would recline all the way in case baby needed it and then the 3 year old of the moment would use the stroller - with the idea of walking if baby needed it. There was only one trip when baby needed it all and then only short times.
Dad not riding coasters shouldn't be an issue - it means you wouldn't do rider swap. You would go with the two 8 year olds and then meet up later. As for splitting up, we toured the parks together but would split up very briefly at times. (For example, doing rider swap and going to Test Track while the other parent takes the younger child/children to the Innovations building or the misting station nearby and then meeting right back up. I don't own a cell phone so that was a another reason to just split up during a ride that the younger child/children wouldn't do. The exception to this was going back out at night. One parent would stay in with the younger children and the other would go back out with the older one when he wanted to go back out. Otherwise, we toured as a family. We've had many trips to both Disney and Universal over the past 9 years and only splitting up briefly works quite well for us.
