Not only will it be an adjustment for your three new children, but your bio-children will also go through a period of adjustment as their roles change. It will not be an easy adjustment and there will probably be some serious knock-down-drag-out fights. (We just had one in my house
)
There will be jealousy and maybe even some resentment between the kids. No matter how happy and excited your kids are about getting new siblings it's a whole different ballgame when the new kids move in and try figure out how they fit in.
My oldest daughter is 11 and my newly adopted son is 10. Although they do love each other now, it's taken two years for us to get there.
I think the most important thing and probably the hardest thing will be to make sure everyone has their own space or at the least someplace to go to be alone.
I've also had a lot of luck pairing an older child with a younger child for activities and chores.
Best of luck to you and your family,
Kathy